Ttyjukl

The closest we get to an exact date comes from Harry Potter and the Goblet of Fire. At least in that year the last day of term is most likely July 1st.

The third task takes place on the twenty-fourth of June:

"The third and final task will take place at dusk on the twenty-fourth of June," continued Bagman.
_{Harry Potter and the Goblet of Fire, Chapter Twenty-Six - The Second Task}

Later on, we find out that the third task (the twenty-fourth of June) is a week before the end of term:

The mood in the castle as they entered June became excited and tense again. Everyone was looking forward to the third task, which would take place a week before the end of term.
_{>Harry Potter and the Goblet of Fire, Chapter Thirty-One - The Third Task (emphasis mine)}

A week exactly from June 24th would be July 1st. It's possible that "a week" doesn't mean exactly seven days, though, and instead means a number of days that's close to a week, so there's potentially some variance on that date.

In The Prisoner of Azkaban, it is noted that on the eve of his 13th birthday (31 July) Harry had not heard 'from any of his wizarding friends for five long weeks'. This would mean that they broke up for summer around Friday 25 June 1993 at the end of Chamber of Secrets. With a return to school date of 1st September, the students will have had a summer holiday that was 9 weeks and two days long. Lucky them!

There is a good discussion of this on the mugglenet forums which may help.

To summarize from there and my own research:

• the wiki claims that term ends in the third week of June, and that the summer holiday lasts nine weeks. No source is given.


• the lexicon claims that term ends in the third or fourth week of June, but also omits a source.


• all of the students are present for Dumbledore's death on June 30. They are also present for his funeral, but they may have been kept late for this as per the quote in HBP29.


• Harry celebrates his birthday at least two weeks after the end of term in HBP3.

From these, the term probably ends in the last few days of June or the first few of July. The description of Harry's birthday shows it to be earlier than July 16, but Dumbledore's death shows it to be later than June 30. The term may end later in book six to accommodate Dumbledore's funeral.

(Skip to the last paragraph in bold if you want the answer without the derivation. It may be helpful, though it is by no means necessary, to first read my answers here and here.)

There are a few dates in June that are explicitly mentioned in the books:

• 
  

  June 1st

  
  
  

  The first day of Exams in Chamber of Secrets (had they not been canceled):

  
  
  

  
  

  Ten minutes into the class, Professor McGonagall told them that their
  exams would start on the first of June, one week from today.

  
  

  
  


• 
  

  June 6th

  
  
  

  The last day of exams in Prisoner of Azkaban:

  
  
  

  
  

  “It’s from Hagrid,” said Harry, ripping the note open. “Buckbeak’s
  appeal — it’s set for the sixth.”

  
  
  

  “That’s the day we finish our exams,” said Hermione, still looking
  everywhere for her Arithmancy book.

  
  

  
  


• 
  

  June 13th

  
  
  

  The term is still ongoing in the year Tom Riddle opened the Chamber of Secrets:

  
  
  

  
  

  The pages of the diary began to blow as though caught in a high wind,
  stopping halfway through the month of June. Mouth hanging open, Harry
  saw that the little square for June thirteenth seemed to have turned
  into a minuscule television screen. His hands trembling slightly, he
  raised the book to press his eye against the little window, and
  before he knew what was happening, he was tilting forward; the window
  was widening, he felt his body leave his bed, and he was pitched
  headfirst through the opening in the page, into a whirl of color and
  shadow.

  
  

  
  
  
  
  
  

  
  

  “Precisely,” said the headmaster. “My dear boy, you must see how
  foolish it would be of me to allow you to remain at the castle when
  term ends. Particularly in light of the recent tragedy . . . the
  death of that poor little girl. ... You will be safer by far at your
  orphanage. As a matter of fact, the Ministry of Magic is even now
  talking about closing the school. We are no nearer locating the — er
  — source of all this unpleasantness. ...”

  
  

  
  


• 
  

  June 24th

  
  
  

  The last day of exams in Goblet of Fire:

  
  
  

  
  

  “The third and final task will take place at dusk on the
  twenty-fourth of June,” continued Bagman.

  
  

  
  
  
  
  
  

  
  

  Ron and Hermione were supposed to be studying for their exams, which
  would finish on the day of the third task, but they were putting most
  of their efforts into helping Harry prepare.

  
  

  
  

While none of the explicitly mentioned dates are the end of term, we can approximate the end of term using various bits of information scattered through the books.

End of term in Philosopher's Stone

The first chapter of Chamber of Secrets takes place on Harry's birthday (July 31st). There are several references there to Harry's confrontation with Quirrel/Voldemort having been weeks earlier:

Harry had slipped through Voldemort’s clutches for a second time, but
it had been a narrow escape, and even now, weeks later, Harry kept
waking in the night, drenched in cold sweat, wondering where
Voldemort was now, remembering his livid face, his wide, mad eyes —

“Dobby heard tell,” he said hoarsely, “that Harry Potter met the Dark
Lord for a second time, just weeks ago ... that Harry Potter escaped
yet again.”

This doesn't tell us how many weeks it has been. However, we know from another passage that it is more than two weeks:

For the first couple of weeks back, Harry had enjoyed muttering
nonsense words under his breath and watching Dudley tearing out of
the room as fast as his fat legs would carry him. But the long
silence from Ron and Hermione had made Harry feel so cut off from the
magical world that even taunting Dudley had lost its appeal — and now
Ron and Hermione had forgotten his birthday.

These quotes are not really enough to narrow down a precise end of term (it could be referring to anything like 3-6 weeks earlier which could put the end of the term anywhere from mid-June to early July.

End of term in Chamber of Secrets

In the beginning of Prisoner of Azkaban we are told that Harry received a phone call from Ron one week into vacation:

Harry was particularly keen to avoid trouble with his aunt and uncle
at the moment, as they were already in an especially bad mood with
him, all because he’d received a telephone call from a fellow wizard
one week into the school vacation.

Later we are told that Harry had not heard from his friends in five weeks:

So Harry had had no word from any of his wizarding friends for five
long weeks, and this summer was turning out to be almost as bad as
the last one.

This reminiscence occurs as Harry turns 13 (i.e. on July 31st):

It was one o’clock in the morning. Harry’s stomach gave a funny jolt.
He had been thirteen years old, without realizing it, for a whole
hour.

If we count the five weeks as starting from Ron's phone call then Harry's birthday would have been six weeks after the term in Chamber of Secrets had ended, giving us an end date of sometime in mid June (which would be consistent with the exam timetable mentioned above).

End of term in Prisoner of Azkaban

In the beginning of Goblet of Fire when Harry's scar hurts, it is described as a fortnight before going back to Hogwarts:

But there was still a fortnight to go before he went back to school.

This puts us in mid-August. When Harry then decides to tell Sirius about his scar hurting, we are told that he had only found out about Sirius being his godfather two months earlier¹:

But then, perhaps it wasn’t so surprising — after all, he had only
found out that Sirius was his godfather two months ago.

This would mean that the events at the end of Prisoner of Azkaban occurred around mid-June.

End of term in Goblet of Fire

In Goblet of Fire the term must have ended just about the last day of June or the first day of July, because it is twice described as going on for one week after the last exam (which as mentioned above was on June 24th):

Everyone was looking forward to the third task, which would take
place a week before the end of term.

A week ago, a student was taken from our midst.

End of term in Order of the Phoenix

In Order of the Phoenix we are told of the O.W.L.s in the beginning of June:

June had arrived, but to the fifth years this meant only one thing:
Their O.W.L.s were upon them at last.

They received their examination schedules and details of the procedure
for O.W.L.s during their next Transfiguration lesson.

“As you can see,” Professor McGonagall told the class while they
copied down the dates and times of their exams from the blackboard,
“your O.W.L.s are spread over two successive weeks.

Their first exam, Theory of Charms, was scheduled for Monday morning.

It can't have been right in the beginning of June, because they didn't get their schedules until "their next Transfiguration lesson" and as the first exam was on a Monday morning, it would have to have been the next week already. The exams going for two weeks means that the last exam would have been shortly after mid-June.

The last exam was on a Thursday (the day they went to the Ministry):

Details of the events that led to the Ministry turnaround are still
hazy, though it is believed that He-Who-Must-Not-Be-Named and a select
band of followers (known as Death Eaters) gained entry to the Ministry
of Magic itself on Thursday evening.

And the following Sunday there were still several days left to the term:

The castle seemed very quiet even for a Sunday. Everybody was clearly
out in the sunny grounds, enjoying the end of their exams and the
prospect of a last few days of term unhampered by studying or
homework.

This indicates that the term continued for about a week after the exams, i.e. it ended with about a week still remaining in June. Indeed we are later told that by the train ride home a week had elapsed since Harry had last seen Sirius, which was the day of the last exam:

The week that had elapsed since he had last seen Sirius seemed to
have lasted much, much longer:

In the beginning of Half-Blood Prince there are three references that indicate that Dumbledore brought Harry to the Burrow two weeks after the end of the previous term. Harry's thoughts about being picked up are:

It just seemed too good to be true that he was going to be rescued
from the Dursleys after a mere fortnight of their company.

When they arrive at the village that Slughorn was staying in we find the following description:

The odd chill that had lain over Privet Drive for two weeks persisted
here too.

And when Dumbledore gave Harry his parting words he said:

"And now, Harry, on a closely related subject... I gather that you
have been taking the Daily Prophet over the last two weeks?"

Additionally, there are several references that point to the day of the last exam (when Sirius died and Harry heard the prophecy) being a few weeks prior to this day (of going to the Burrow).

Slughorn tells him:

Ever heard of Sirius Black? You must have done – been in the papers
for the last couple of years – died a few weeks ago –"

Upon departing from Slughorn Harry recalls the words of the prophecy:

He was reminded of words he had heard a few weeks ago, words that had
a horrible and particular meaning to him: Neither can live while the
other survives ...

When Harry sees his O.W.L results the next morning he again thinks back to the prophecy:

Moreover, it had seemed the right destiny for him since he had heard
the prophecy a few weeks ago.

The term "a few weeks" would seem to indicate at least more than two weeks. As established above, the day Harry heard the prophecy was one week before the end of term, so it would have been three weeks prior to Harry coming to the Burrow which would be consistent with the use of "a few weeks".

However, on the day they receive their O.W.L. results (the morning after Harry arrives at the Burrow) they are speculating as to when they would receive them, and we have the following quote:

"Can't be long now, it's been a month," said Ron.

This seems to imply that the exams were completed a month earlier, which would make this a week later than the three weeks mentioned above. Of course, the various uses of "weeks" or "month" are not necessarily perfectly precise, so it may have been something in between.

End of term in Half-Blood Prince

In the beginning of Deathly Hallows Harry reads what is described as "today's Prophet":

Harry moved across the room, slid the mirror fragment off today’s
Prophet, and unfolded the newspaper.

In that newspaper we find the following:

Her nine-hundred-page book was completed a mere four weeks after
Dumbledore’s mysterious death in June.

This takes place on the day Harry leaves Privet Drive, so this would tell us that he arrived at the Burrow approximately four weeks after Dumbledore died.

We are next told of a conversation that took place on the third night of Harry's stay at the Burrow:

“I think Mum thinks that if she can stop the three of you getting
together and planning, she’ll be able to delay you leaving,” Ginny
told Harry in an undertone, as they laid the table for dinner on the
third night of his stay.

At the end of that dinner we are told that the Delacours would be arriving the next morning:

“No, Harry, dear, I’d much rather you helped Arthur muck out the
chickens, and Hermione, I’d be ever so grateful if you’d change the
sheets for Monsieur and Madame Delacour; you know they’re arriving at
eleven tomorrow morning.”

We know that the Delacours came two days before the wedding:

“Ginny’s probably left a speck of dust on a poxy napkin ring,” said
Ron. “I dunno why the Delacours have got to come two days before the
wedding.”

And we know that they came one day before Harry's birthday because the day after his birthday is the day of the wedding:

Three o’clock on the following afternoon found Harry, Ron, Fred, and
George standing outside the great white marquee in the orchard,
awaiting the arrival of the wedding guests.

As Harry birthday is July 31st, the Delacours must have arrived on July 30th. The conversation with Ginny must then have been on July 29th, which means that July 27th was the day that Harry read "today's Prophet" in which Dumbledore's death was described as having occurred 4 weeks previously.²

This means that Dumbledore died on one of the last days of June. This can be further demonstrated from the trio's meeting with Scrimgeour during Harry's birthday meal. Harry says:

"Dumbledore died over a month ago. Why has it taken this long to give
us what he left us?"

Scrimgeour's justification was a law that allows the Ministry to confiscate the contents of a will. Hermione points out that the law only allows the Ministry 31 days for this:

"No, it'll be because the thirty-one days are up," said Hermione at
once. "They can't keep the objects longer than that unless they can
prove they're dangerous. Right?"

If the 31 days are up on July 31st then Dumbledore must have died on June 30th.³

After Dumbledore's death in Half-Blood Prince we are told that all lessons and exams were canceled:

All lessons were suspended, all examinations postponed.

This implies that exams had not even started yet, and certainly that they had not been completed yet, despite it already being the last day of June. As exams normally go on for a week (or two weeks for O.W.L.s), and there is usually a last bit of term after exams, this would indicate that the term was scheduled to continue at least halfway into July.

Further evidence that exams had not started yet comes from Harry's relationship with Ginny:

Ginny’s O.W.L.s were approaching and she was therefore forced to
study for hours into the night. On one such evening, when Ginny had
retired to the library, and Harry was sitting beside the window in
the common room, supposedly finishing his Herbology homework but in
reality reliving a particularly happy hour he had spent down by the
lake with Ginny at lunchtime, Hermione dropped into the seat between
him and Ron with an unpleasantly purposeful look on her face.

Thus, the setting here is one evening during the time period that Ginny was preparing for O.W.L.s. Following the conversation with Hermione, Harry is informed that Dumbledore wants to see him:

Harry was shaken from these bitter reflections by the appearance at
his side of Jimmy Peakes, who was holding out a scroll of parchment.

“Thanks, Jimmy ... Hey, it’s from Dumbledore!” said Harry excitedly,
unrolling the parchment and scanning it. “He wants me to go to his
office as quick as I can!”

Harry and Dumbledore, of course, find the Horcrux and Dumbledore dies. Thus, the day Dumbledore died was during the time period when Ginny was still preparing for her O.W.L.s (i.e. they had not yet started).

Additionally, in the same conversation Harry mentions his detentions with Snape:

“You still got detention with Snape this Saturday?” Ron continued.

“Yeah, and the Saturday after that, and the Saturday after that,”
sighed Harry. “And he’s hinting now that if I don’t get all the boxes
done by the end of term, we’ll carry on next year.”

This tells us that the term was scheduled to continue for more than two additional weeks, which (per the above calculations) takes us past mid-July.⁴

Thus, it appears that there is no set date for the end of term. Fifty years prior to Chamber of Secrets the term ended some time after June 13th. In Philosopher's Stone we don't know when the term ended. In Chamber of Secrets and Prisoner of Azkaban it seems to have ended around mid-June. In Goblet of Fire it seems to have ended at the very end of June or the very beginning of July. In Order of the Phoenix it appears to have ended about a week before the end of June. In Half-Blood Prince it appears to have extended all the way to mid-July.

^{1. Though this is what is stated in Goblet of Fire, it is technically incorrect. Harry actually found out that Sirius was his godfather well before Sirius told him. He overheard Fudge mention it when he was in the Three Broomsticks:}

^{“Of course they were,” said Fudge. “Potter trusted Black beyond all
his other friends. Nothing changed when they left school. Black was
best man when James married Lily. Then they named him godfather to
Harry. Harry has no idea, of course. You can imagine how the idea
would torment him.”}

^{Despite this, the point is the same, namely, that Sirius had only become a part of Harry's life two months earlier.}

^{2. There is, perhaps, a slight difficulty with this. We have the following sentence which describes the day that Harry reads the Daily Prophet:}

^{It was stupid, pointless, irritating beyond belief that he still had
four days left of being unable to perform magic...}

^{This took place in the afternoon, so it would seem likely that that day is not being counted as one of the four days. Since on his birthday he would be able to do magic immediately, this would mean that there were four additional days between the current day and his birthday. That would make the current day actually July 26th, rather than July 27th.}

^{However, one could argue that Harry was including the current day as one of the four (even though the day was already mostly over). Alternatively, one could argue that when the conversation with Ginny was described as occurring on "the third night of his stay it was not counting the night he arrived as the fist night.}

^{The latter possibility, though, is much harder to argue because the night that Harry arrived is explicitly referred to as "the first night" elsewhere:}

^{All the jobs she handed out seemed to keep him, Ron, and Hermione away
from one another; he had not had a chance to speak to the two of them
alone since the first night, when he had told them about Voldemort
torturing Ollivander.}

^{Additionally, there is further evidence that Harry arrived at the Burrow on the night of July 26th. The morning after arriving Harry tells Ron:}

^{"The Trace'll break on the thirty-first," said Harry. "That means I
only need to stay here four days. Then I can –"}

^{The four days that the Trace would be operative for would then be July 27th, July 28th, July 29th, and July 30th. That would mean that this conversation was taking place on the morning of July 27th, which means that Harry arrived on the night of July 26th.}

^{However, it is hard to know what to make of this because Ron's response doesn't really add up:}

^{"Five days," Ron corrected him firmly. "We've got to stay for the
wedding. They'll kill us if we miss it."}

^{As the wedding was the day after Harry's birthday (August 1st) this would actually have required Harry staying for six days, not five. The only way around this would be to argue that for some reason Harry was including July 31st in the four days that he would have to stay, in which case staying for the wedding would indeed only require a fifth day, which would mean that Harry had in fact arrived on the night of July 27th.}

^{3. It is actually a bit more complicated than this. The description of Rita's book as having been completed four weeks after Dumbledore's death doesn't necessarily mean that the Daily Prophet article is also from four weeks after his death. It is possible that the book had already been completed prior to the article, in which case the article might have been written, say, five weeks after Dumbledore's death (which would make the date of his death a week earlier). In fact, the day of the Daily Prophet article had to have been more than four weeks after Dumbledore's death, for the following reason:}

^{We know that Harry had already been home for at least four weeks from the following passage:}

^{Vernon Dursley had been changing his mind every twenty-four hours for
the past four weeks, packing and unpacking and repacking the car with
every change of heart.}

^{We know that the Hogwarts Express had left on the day of Dumbledore's funeral:}

^{Harry rose early to pack the next day; the Hogwarts Express would be
leaving an hour after the funeral.}

^{And we know that the the funeral was at least three days after Dumbledore's death, based on the following:}

^{Some students were hurried away from Hogwarts by their parents over
the next couple of days}

^{This tells us that there were at least two days after the death, plus "the next day" (from the previous quote). This would mean that Dumbledore's death was at least 31 days before Harry left Privet Drive (28 + 3) which means that the latest it could have been would be June 27th.}

^{4. And according to the contradictory evidence cited in the previous footnote, the end of term would have been at least three days earlier.}

^{Additionally, we can throw in another complication based on further contradictory evidence. The evidence from Ginny's exams might actually indicate that there was a lot more than three days between Dumbledore's death and his funeral. During the in-between period we have the following narrative:}

^{The beautiful weather seemed to mock them; Harry could imagine how it
would have been if Dumbledore had not died, and they had had this time
together at the very end of the year, Ginny's examinations finished,
the pressure of homework lifted...}

^{This tells us that at some point before Dumbledore's funeral they had already reached the point where Ginny's exams would have been finished (had they not been canceled). If Ginny's exams hadn't even started yet when Dumbledore had died, this would mean that this was already two weeks later and the funeral had still not yet occurred. If this is true then Dumbledore's death would have had to have been at least two weeks prior to his funeral, and the funeral was after the point when the exams would have ended. Therefore, we no longer have the additional two weeks to add on to the term as those two weeks already occurred between the death and the funeral (which could have been no later than June 30th for there to have been four weeks for Harry to be home before the day he left). This would make the end of term some two weeks earlier than the calculations in the main text of this answer.}