We went the week before Christmas two years ago, and when we first arrived, it wasn't too bad. The parks weren't dead by any means, but not packed either. As the week went on, however, the crowds picked up and by the 21st, 22nd, it was extremely crowded. This is also exactly what I was expecting, as I had read the exact thing on another Disney Board. Since you're leaving right before Christmas, I think you'll be fine for at least the first half of your trip, if not longer. The crowds will get really big about the same day you're leaving or the day before.