I know statistically early December is the slowest time of the year, but from my research and personal experience I have NOT found that to be true. I went last December on December 12 and it was very crowded. Waits for FL rides mid day were 30 + minutes. They were much longer for stand by lines for the major rides.
In 2005 I was there in early Feb, mid April, mid Sep and the one day in December. I think December was the busiest followed by April. Both early Feb and mid Sep were very uncrowded. To me, those are the least busy times. (I just returned this past week, May 2-4, and it was not busy either).
People are reading that early Dec is not busy, so it seems that everyone is going then. It's not the empty time of year it used to be. Last year the crowds didn't rival summer or spring break, but it was not slow.
Many schools may get out on Dec 15 for their holiday break. I'm guessing that crowds will pick up dramatically on that day.
Have a great trip!