We were there during Christmas Week in 2014 and it was amazing.... and busy.
Christmas Eve gets packed closer to the evening and Christmas Day is pretty full at MK and EPCOT. It took us 90 minutes to get from Animal Kingdom Lodge to EPCOT on Christmas morning -- so when you make your plans:
- Choose Animal Kingdom on Christmas Day or get to EPCOT super early on Christmas morning. I wouldn't try Magic Kingdom - it'll be full super fast.
- Stay on property but choose a hotel that's centrally located to most of the parks.
- Plan your days so that you do more of the busier parks further away from Christmas. If you're there the 21-28, try to do MK as close to the beginning as possible - or the day after Christmas and further out in your trip to departure day.
- Make your Fast Pass reservations for LATER in the day - not in the morning. Same goes for ADRs, do lunch. That way if you do get delayed you're not penalized. (We missed Test Track FP+ by 5 minutes due to the 90 minute bus ride to EPCOT).