I think Disney is expecting it to be a very busy December. There have been quite a few changes to the park opening and closing times, seeing all 4 parks with extended hours. It would seem that even Animal Kingdom will be open from 8am - 9pm some days.
The Magic Kingdom's hours have changed a lot and on some days is open from 8am - 1am and then there are Extra Magic Hours in the morning and at night. This is obviously to do with the New Fantasyland.
You can find the updated hours here