Hi Christine,
The Christmas holiday is definitely one of the busiest times at Disney World. The holiday period between Christmas and New Years is typically a capacity crowd and the lines and waits will be at a maximum.
We have visited a number of times within the first two weeks of December and have always found the crowds at a moderate level. I like to use
www.touringplans.com to help sort out crowd levels. The site uses a number of factors like holidays, past attendance and special events to rate each day on a scale of 1 to 10. 1 is the smallest crowd and shortest wait times and 10 is the highest.
With Christmas falling on a Friday this year, the Touring Plans calendar is indicating that the crowds should be expected to increase the week of Christmas, beginning around the 19th, and continuing through New Years. At this time the crowd level is at a 10 and the crowds will be large, the waits high and you may experience times that a park closes temporarily due to capacity levels. Prior to the 19th, the crowds are averaging a 4 to 5 and the waits will be reasonable with higher waits for the more popular attractions.
If you are trying to decide a travel time and are not limited by school or work, I personally would select dates within the first two to three weeks of December and not after Christmas. With the earlier travel dates you will still be able to enjoy all of the holiday decorations and special events and miss a majority of the crowds.
Hope you have a wonderful time. That is my absolute favorite time of the year to visit Disney!