We were there in 2002 from the Friday before Christmas (Christmas Day was the following Wednesday) to the Saturday after. The weekend before was fairly crowded but as we got closer to Christmas Day, the crowds started increasing and stayed that way until we left.
The week of Christmas and New Years are probably the most crowded times of the year.
Let's put it this way, we very much enjoyed being able to experience the holiday season at WDW, but we have absolutely no desire to return during that time of year again, except maybe at the beginning of December if given the chance.
I simply cannot stand the crowds!!!