Where possible you should always try and avoid the American School Holidays. If you don't then you are going to be spending 90% of your day standing in lines. In American there are no set dates for there school holidays. The 2 main ones you want to avoid are the Spring and Summer breaks. We hit the parks at the start of April which was for 2011, between the College Spring Break and the School spring break. But this was more good luck than good management.
Statistically the worst time for crowds at the parks is mid March to mid June. Which is the Spring Break and Mid June to Mid August. which is the Summer Break.
The time of year that has the least crowd numbers is from Thanksgiving day to Christmas. You also get the added bonus in seeing Disney decked out in Christmas decorations.
