Never, because I'm an "adult" now and have to do "adult" things.
Like thinking about maybe getting a job soon while actually sitting on my ass playing Pokemon because I'm still five even though I have a wedding that I need to plan and pay for.
Mine never really ends, not in a strict sense. I'm home schooled, so it isn't unheard of for me to take online summer classes if the subject matter seems interesting.
You weird Northern Hemisphere people :p Summer holidays for schools are from December to February. Now that I'm in uni I get the whole of Feb and March off as well :)