Here is what I found online, I don't really know first hand
busiest on the following days:
* Magic Kingdom: Monday, Thursday, and Saturday
* Epcot: Tuesday and Friday
* Disney-MGM Studios: Sunday and Wednesday
* Disney's Animal Kingdom: Monday, Tuesday, Wednesday"
Early Entry days
Epcot- Wed
MK-TH, Sun
AK-Friday, Mon
MGM-Sat, Tue
Many people also warn Early entry days are very busy, and some here swear the best day at any park is the one following an EE day. Of course the one the busiest days info conflicts with the day after EE for MK....