This is the week we are going, and is about the same time we went several years ago. Now I imagine that it will begin getting crowded by Wednesday, and by Friday it will be very busy, with the weekend being REALLY busy. MK is going to be crowded on Saturday, Monday, Wednesday and again on Saturday because there are no parties those nights. People who want to see the nighttime shows will be there and it is shoulder to shoulder. I would plan MK earlier rather than later in the week on a party day because those days tend to have lighter crowds, however as the week progresses I think it is going to be more crowded, party or no.
I agree with
@ShadeDK regarding the other parks. The party days in M are going to impact the other parks, and I bet you will feel it most in DHS, which is smaller that DAK and Epcot.
The weather is all over the place. I suggest you plan on layers. It can get cold, and since my DH seems to be the outlier in our group when it's cold in December, bring your mittens! LOL! He is joining us this year. Seriously, we have had trips that the weather was warm enough to not only swim but to enjoy the pool area, and trips when there was no way to stay out there.
The holiday season is wonderful in WDW> WE go to the Candlelight procession in Epcot, visit the storytellers in the WS, spend time checking out the decorating details all over every park, and make time to resort hop. Really beautiful!
I remember that the last trip I took that brought us home on Sunday the 22nd was crazy from Friday to Saturday, and Sunday even navigating the resort was frustrating. Mange your expectations and you can have a great trip!