The first week or two are really nice times to go. If at all possible, try to make the trip before the schools start letting out. It's like an overnight transformation when the schools start letting out! Back in my CP days in 2002, one day the parks were kinda slow and the next day you couldn't even hardly walk through the magic kingdom. The week before and the week after Christmas are the worst.
If possible, I'd avoid those weeks. If not, it is still possible to enjoy your trip, just expect crowds and use fast passes wisely. Allow extra time to get from point a to point b and line up early for parades and fireworks. December is a wonderful time to go, because of all the decorations and extra festivities.
Enjoy your trip!