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Rob (DisneyRob.com) gives the following information on Early-Entry Days.
If you are a resort guest you get the privilege of entering the parks before the general public. These are called Early-Entry Days. These are days that the park opens an 1 hour and 1/2 before the official opening of the park. Early entry days are on a rotational basis and take place seven days a week. Many guests who try early-entry are disappointed that the entire park is not open, only specific attractions are operational until official opening time. To avoid disappointment, you will find info on what attractions will be open during early-entry below each park.
Magic Kingdom days -- Monday, Thursday and Saturday.
Rides Open ...
Fantasyland:
Tomorrowland
EPCOT days -- Tuesday and Friday
Attractions open ... SpaceShip Earth Test Track and the Wonders of Life
MGM days -- Wednesday and Sunday
Attractions open ...
Note: there are currently no early-entry days for Animal Kingdom.