It's usually around the 20th of July. Although it can fall a couple of days before that date.

EDIT: To add more. Below are the dates of a Manchester School year for the year 2005/2006. Be aware these can change between different places.

Term Starts Monday 5 September 2005
Half Term Holiday Monday 24 October 2005 -
Friday 28 October 2005
Term Ends Friday 16 December 2005
Spring Term
Term Starts Tuesday 3 January 2006
Half Term Holiday Monday 20 February 2006 -
Friday 24 February 2006
Term Ends Friday 7 April 2006
Summer Term
Term Starts Monday 24 April 2006
Bank Holiday Monday 1 May 2006
Key Stage 2 Tests Monday 8 May 2006 -
Friday 12 May 2006
Key Stage 3 Tests Tuesday 2 May 2006 -
Friday 5 May 2006
Year 7 Progress Tests Monday 8 May 2006 -
Friday 12 May 2006
Half Term Holiday Monday 29 May 2006 -
Friday 2 June 2006
Term Ends Tuesday 25 July 2006

 

I added more to my last post that you might want to look at.

 

Hogwart's isn't part of the muggle state system though.

According to hp-lexicon, which has timelines for each year, the date the Hogwart's Express takes the students back to London is not fixed:

First year: June 20th
Second year: June 19th
Third year: June 18th
Fourth year: July 3rd
Fifth and Sixth year they don't seem to know.

 

*stares in horror* July 25?! That's only like a month and five days of vacation!

Plus it screws with plotlines. Nope, I'm happy with sticking to early June/mid-June end of term dates for my fanfiction. (It seems weird that lexicon says they get out on different dates, though. Well, I suppose not. My school does that too, cutting into summer vacation becasue we used too many snow days. *sigh*)

 

I know this is possibly one of the biggest necro posts ever perpetrated here, but oh well. I came across this thread earlier, and then happened to come across a page that discusses it while I was looking up Ravenclaw House online. So here's the answer to a nearly-three year old question:


" Hogwarts' school year is structured in a similar way to other 'non-magical' schools and colleges in the UK, with a three-term year punctuated by holidays at Christmas and Easter and bounded by the long summer holiday of nine weeks. Term begins every year on 1 September, and finishes at the end of June the following year. Students have the option of staying at Hogwarts for the winter and spring holidays. Those who choose to stay at the castle do not have lessons and attend a feast on Christmas Day. Students also do not have classes the week of Easter, but this is much less enjoyable due to the large amount of work that the teachers assign students at this time in preparation for final exams.

Other than the aforementioned breaks, and weekends, students do not receive any other holidays. There are normally four feasts per year, the start-of-term feast at the beginning of the school year and end-of-term feast at the end of the school year, as well as feasts at Halloween and Christmas. Feasts are also called to mark any special occasions, as in Goblet of Fire, when there was a feast to celebrate the beginning of the Triwizard Tournament. "

Found at http://en.wikipedia.org/wiki/Hogwarts