Definately book hotel ASAP and make your dining reservations at your 90 day mark, +10 days if you are staying onsite.
MVMCP (Mickey's Very Merry Christmas Party) will be going on on December 16, 18 and 20. These are the last three days of the party. Don't know if you know about this or not but it is a separate ticket event starting at 7:00 pm in the MK (you can actually get in at 4:00 with a ticket). Basically the park closes at 6:00 (although you do not have to leave) and reopens at 7:00 and there are special Christmas shows, parades, fireworks and it snows on Main Street. Definately something worth doing.
Epcot has the Candlelight Processional. A Disney tradition now in its 35th year (2005), the Candlelight Processional is a moving presentation of the Christmas story in readings by a celebrity narrator and music by a Mass choir and 50-piece orchestra. Narrator - Marlee Matlin Dec 19-21 - To Be Announced Dec 22-27. This event takes place in the America Gardens Theatre in World Showcase in Epcot. It is an outdoor (uncovered, not climate controlled) setting and is not cancelled due to inclement weather. It is recommended you dress accordingly. There are 3 shows nightly 5:00, 6:45 and 8:15. Make sure to stay for IllumiNations - which is at 9:00 each night in Epcot on the lake. Last year they had a special holiday version. At the end of the show they play "Let there be peace on earth" and did a wonderful fireworks show to the music. I do not think there was a dry eye in the place. They did this the whole month of December. Epcot not only has beautiful holiday displays, but also acknowledges the multicultural diversity that makes up our planet by sponsoring "holiday storytellers" in each pavilion around World Showcase. When you enter Epcot you can get a schedule of all the storytellers. Basically it is a short (10 to 15 minute) show in each country about how they celebrate the holidays. When each show is over they stay around for pictures and autographs. We used the picture with Mr and Mrs Clause from the American Adventure as our holiday card last year.
The afternoon parades in MGM and AK are Christmas themed. Another definate to do is visit the different Disney hotel (especially the Deluxe Resorts. Grand Floridian has a life size Gingerbread house in the lobby and a beautiful tree. Animal Kingdom Lodge and Wilderness Lodge are also decorated beautifully. The Boardwalk has an replica of an amusement park made out of candy and chocolate and the Beach Club has a carousel made out of chocolate.
Besides all the above - enjoy the parks. They are decorated for the holidays. We went last year the first week in December and the park was pretty empty. As you get closer to Christmas the park does get busier. Have a wonderful time and sorry if this is so long but the holidays at Disney is just about my favorite thing.
Linda