A couple of things to check.
First what's the AP blackout level. If it's fully blacked out, then they expect a big crowd and want to take full ($$) advantage of the possibilities.
Second check the park hours & show schedules. When they have a pair of Fantasmic shows it's going to be a busy day. When they add a third show it's hell with fireworks.
Finally, think about the school schedules. Most schools will be closed from Dec 20th to Jan 1st, so that means vacationing families.
A couple of things to remember on even on the most crowded days:
1. Early (pre-11am) and late (after the fireworks) usually provide the smallest lines.
2. Take full advantage of fastpass and don't be in a hurry for anything
3. Now's the time to do those things you usually skip like the Main Street Movies, Innoventions, HISTA and the 50th Movie
4. Visit DCA, it'll be more crowded than usual but probably quite bearable, the Aladdin Show is top notch and they'll have 2 parades (MSEP & BPB) during the busy season.
5. Eat outside of DL, try Rain Forrest Cafe, ESPN Zone, Ralph Brennans or House of Blues in DTD or Ariel's Grotto or Taste Pilots Grill in DCA.
6. The latest shows are the least crowded, so use the earlier parade & showtimes to do things away from the parade route / show area.