Below is a listing of which parks are busiest on which days as posted by Safari Steve, a CM who posts here. We base our schedule on this, hitting the parks early then hopping to Epcot when crowds get heavier, usually about 1 pm.
Sunday is historically the slowest day at MK, with Wednesday a close second. Sunday is slow at all four parks as it is a MAJOR travel day. Epcot is slow on Mondays, as most guests arrive on Monday and want to spend their first day at MK. Tuesday is Epcot's first heavy day of the week. Tuesday and Wednesday are heavy at MGM, but Sunday and Monday are slower. AK is still hard to judge as it is still a baby and still defies most attempts to identify patterns (just like a two to three year old human) Fridays and Saturdays are heavy at MK, but not as heavy as Mondays. Generally, Saturdays are heavier at all four, and all of these generalities are to be totally ignored during peak days and holidays (New Year's Eve was on a Sunday... we closed the gates before 10am) Good luck!
Bill From PA