I got a WDW book at Books a million. There's a ton of them w/info about what days to go to parks. Here's a list from one of my books.
Busiest days in the park:
Monday: MK, AK
Tuesday: MK, EP
Wednesday: EP
Thursday:MK, AK
Friday: EP, MGM
Saturday: MK, EP
Sunday: MGM
Last year I went in March (during everyone's spring break)!!! But, we went Sunday and it was great. There was only a 15 min wait for space mountain at 4 o'clock. Hardly any waits on any ride. We also went back Monday. Monday morning was great. We did the whole left side of the park (POC, Aladdin, Splash, Big Thunder Mountain, Haunted Mansion, and Jungle Cruise) before 12:00!!!!! We even got to ride Big Thunder Railroad twice in a row. But after noon, the crowds were BAD!!!!! Hundreds of people, esp. in the kiddie areas (pooh, snow white, dumbo, etc) 1 hour waits. Also, we went to AK on Tuesday and the crowds were bad!!!!!...but with fastpass we got to do everything.