I do not remember what sight I got this from but here is the information they gave. I got this just last month.
BEST DAYS TO VISIT PARKS
The best and worst days for the Magic Kingdom are:
Magic Kingdom
Highest Waits Thursday
Sunday
Monday
Friday
Wednesday
Saturday
Lowest Waits Tuesday
A couple of things to note: Days with morning Extra Magic Hour were more crowded than other days of the week. Thursday is the most common day for Morning EMHs, so Thursday is usually the day with the highest waits. Sunday is also a busy day. Tuesday, Saturday and Wednesday (when they don't have EMHs) are the best days of the week to visit the Magic Kingdom.
A good rule of thumb seems to be to visit the park the day after its morning EMH (not, as one might expect, its evening EMH).
The best and worst days for Epcot are:
Epcot
Highest Waits Tuesday
Sunday
Monday
Saturday
Friday
Thursday
Lowest Waits Wednesday
Disney Studios
Highest Waits Friday
Sunday
Thursday
Wednesday
Monday
Tuesday
Lowest Waits Saturday
Animal Kingdom
Highest Waits Monday
Sunday
Tuesday
Saturday
Thursday
Wednesday
Lowest Waits Friday
For Disney's Water Parks:
Water Parks
Highest Waits Thursday
Monday
Tuesday
Friday
Wednesday
Sunday
Lowest Waits Saturday
For Universal Studios:
Universal Studios
Highest Waits Thursday /
Monday
Sunday
Wednesday
Saturday
Tuesday /
Friday
Lowest Waits
For Islands of Adventure, we think it's something like the following (we may need to go count more people):
IOA
Highest Waits Thursday
Sunday
Wednesday
Friday
Saturday
Monday
Lowest Waits Tuesday