The average high temp. in 73, low temp.50. But it could get into low 80s and drop closer to freezing. It's one of those situations when you should plan to pack shorts and pants and layers for your tops. Check the weeks forcast just before you leave. That way you'll know if there is a cold front coming through or not.
Christmas week will be crazy busy. Just plan to ride your favorite rides first thing in the mornings. Then you'll have the afternoon and evenings to enjoy the special holiday events, without thinking about trying to get into long lines for rides.
Specialty events during that week include:
Epcot: Candlelight Processional--make reservations now for dinner package which includes guaranteed entry into this evening event. Also, Holidays Around the World and Christmas tree lighting in Future World
MGM: Osbourne Family Light display--go see it during one of the Fantasmic showings and it won't be quite so crowded.
Magic Kingdom: I think Mickey's Very Merry Christmas Party will be over by the time you arrive....it's a ticket only event anyway.
Every park has later than normal hours because of the holiday week. We're also planning time to visit various resorts to see Christmas decorations.
Have fun, but be prepared for large crowds!
