Disney isn't that busy until right before Christmas. And with Christmas Eve falling on a Friday, I doubt too many people will even be there until that day or maybe the day before. It's always busiest when kids are out of school (i.e. from Christmas to New Year's).
And you're better off going before Christmas than after it; I've gone down on New Year's Eve, which is cool 'cause it's crazy-busy and you get to enjoy the New Year's Eve festivities and then the parks empty out for the first week of January, but they also tend to shut major rides for maintenance. I've literally gone in on 1/1, ridden something cool like
Pirates,
Splash, or
Haunted, and come back on 1/2 to find it shut down for maintenance for the rest of the week.
