First if you are actually intending to do it this year, you should make the ressie immediately. That time fills quickly once the 11 month window begins. It is hugely crowded. That crowd actually starts to build week before Christmas and reaches huge about the 22d and stays there until after Jan 1. When Jan 1 falls determines how soon crowds go. Next year it is a Thursday, so large crowds will still be around until Sat, the 3d, and then diminish quickly to real pleasant by Jan 5.
You get park closings to new entries because it is full. Stand-by lines for popular rides go to well over 2 hours (3 for Test Track and Soarin) and they essentially stay at that level from 10:30 to almost closing. Secondary attractions go to over an hour. You must have ressies for dinner far in advance if you want to eat. Street crowds are enormous, that 15 minute stroll from Space Mountain to Splash Mountain during slow season becomes 40 to 45 minutes because of crowd size in the street. Early mornings you can do some things quickly, but otherwise fast pass is needed and they run out for anything popular by 11:30.
In return (and you can actually get these anytime in December so it is not necessary to go at Christmas), you get some amazing Christmas decorations at parks, hotels and everywhere else. The Osborne lights at the Studios is just incredible. Country Bear Jamboree switches to its Christmas show. It is complete Christmas overdose and you will love it. Just don't go expecting to actually do many rides.