Almost daily.
That's funny.
This is a little confusing, 'cause some people are quoting breaks on the year-round system (which I know some osteopathic schools use - I don't know which allopathic schools use it) and some are quoting breaks on the traditional semester system.
My school uses the traditional semester system. We start a little earlier than the other schools on the health sciences campus because, since our exam periods are a week long with no other classes, we run on an 18-week rather than a 16-week semester.
We get almost a month off at Christmas - about the 15th of December to the 10th of January. Spring Break is a week at the end of March; although - unlike traditional Spring Break after mid-terms, our Spring Break works out to be 2 weeks before block exams, so I was studying by the pool in Palm Springs. Thanksgiving is 4 days off although, unless you're super-organized, it's not a 4-day holiday - more like 2 days and the other 2 devoted to catch-up study.
1st year is the only summer off. 2nd year you'll be studying for Step 1 from the end of classes until your test date in June, and then it's into the wards July 1.
Medical school is indeed a lifestyle, and not really filled with vacations, unfortunately. Just look at this way, though: once you're a medical student, you'll get to experience partying like it's 1999 three times a semester on the weekend after test block.