With one of my favorites coming up, I thought this would be a good time for this one.
Though Halloween is pretty high on the list, I'd have to go with Independence Day. It's during the summer, you get a day off for it, there's a damn good reason for the holiday, and you get to play with explosives while drinking. How much more fun can you ask for?
Though I should mention, my favorite holiday hands down would be Oktoberfest if I lived in a country that celebrated it appropriately.