Hi Stacy!
Well, there are 2 different answers for your question---
First of all, the Disney website has a great calendar that has all the times of park opening/closing and shows and special events. Just click PARKS above, then click the park you want to go to and then CALENDAR. You can browse the calendar by day and see all the hours including Extra Magic Hours!
There is another website called TouringPlans.com that will give you crowd predictions for the days you are at the parks. Then you can decide which one to go on a given day. However, it is a subscription based service, so you'd have to pay to see all the details. But it IS fascinating to look at. (And if you google "crowd prediction Disney World" you'll probably find some good insight, too!)
Disney does have
a good list of when to go to which park, too. For example, they say the busist days are:
Magic Kingdom Park: Monday, Thursday, and Saturday
Epcot: Tuesday and Friday
Disney's Hollywood Studios: Sunday and Wednesday
Disney's Animal Kingdom Theme Park: Monday, Tuesday and
Wednesday
Hope that helps!