I can't remember the exact order...but I do know that in general, Mondays tend to be the busiest for MK and Tuesdays tend to be the busiest for Epcot. Since most people arrive at their vacation destination on a Sunday, they are more likely to spend their first full day (Monday) at the more popular location. Most people like to start off their first full day at MK (not true for everyone, but for a good number of people it is). Epcot is usually the next park of choice for many guests, hence Tuesdays being the busiest for them. Now this is generally speaking...there will always be exceptions. I can't remember the order of the other days and the other parks.