I wrote a free guide about Christmas. You can find it at survivedisxmas.com
You can pretty much expect all the parks to be crowded during Christmas week. There's no real escaping the crowds.
While my family usually heads down on Christmas Eve, I've heard that the crowd "dies down" a bit between the last day of
MVMCP and Christmas Day. December 21st through the 23rd should give you a few days of lower crowd levels - although that may change due to the free dining. You'll see the crowds pick up on December 23rd and 24th as many will arrive for Christmas.
This year will be interesting. I'm trying to remember how bad the crowds were for our first trip. That was the last time Christmas fell on a weekend. I think locals may be more inclined to stay a night if they have off the next day. This could lead to more crowds, higher demand for ADRs and less availability at the resorts - but again, that's just my opinion. I think you'll see the crowds really pick up on December 25th and 26th, and die down on the 27th and 28th when people return to work.
Most people visit WDW for Christmas OR NYE. We stay the full 10 days. We usually see people checking out by the 27th. It's not too bad on the 28th and 29th, and then it gets chaotic again starting on the 30th.
We usually do Magic Kingdom on Christmas Day. We're crazy, right? Not as crazy as you'd think. Magic Kingdom should open to the public at 8am on Christmas Day, with a morning EMH at 7am. We always try and get to MK for the morning EMH, but usually only get there for the last half hour. When we arrive, Space Mountain is about a 20-minute wait. I know this because that's usually where we head to first. After, we grab breakfast (a cinnamon roll in past years, but this year we plan on changing it up) and then head to a popular ride with a low wait time. Usually the Jingle Cruise. You'll see the crowds pick up at 8am, but it's nothing crazy. It does get crazy around 10am. My theory is that most people spend the morning eating breakfast, opening gifts and most locals wait to find a parking spot. By 11am, you will see a phased closure.
Animal Kingdom is probably your best bet for Christmas Day and usually sees the lowest crowds - comparatively speaking.
It's tough to say until EMHs come out. Magic Kingdom usually does late-night EMH the Friday after Christmas. Well, this year Christmas falls ON a Friday. I can't see Magic Kingdom doing late-night EMH on Christmas. They'll probably close by 12am or 1am, honestly. You may see the late night EMH on Sunday, December 27th.
We travel on December 24th and wake up early on Christmas. We usually rest a bit on December 26th and head to the parks a little later - like 3pm. We'll usually do EPCOT which works well for us. Yes, the rides have high wait times by the time we get there, but we schedule out FP+s accordingly, so we never have a problem. EPCOT is a nice change of pace. We go from Magic Kingdom on Christmas Day - where we need to "ride and see everything" while fighting the crowds, so EPCOT - where the crowds are more spaced out, there's only two or three rides to go on, and you can move at a much slower pace. EPCOT is so much more relaxing for us - nothing beats touring the countries and eating and drinking.
Another thing my family does is take breaks in the afternoon. The parks are very crowded by 1pm. We usually call it a day by 1 or 2pm and head back to the resort. I'll go for a jog, take a nap, or just relax by the pool. We'll shower and head to dinner around 5pm, and then back to the parks to end the day. We find that this is a nice break. We'll head back to MK for the last parade or last showing of Wishes. Many of the locals and young families head home or back to the resort after the first parade/fireworks show. By taking a break in the afternoon and heading back after some of the crowd has left, we have a much less stressful day.
Make sure you get the Park Hopper option. It's a small cost in the scheme of things (your free meal plan is a solid trade off.) If a park is way too crowded, it gives you the freedom to leave and go to another park.
I'll be posting blogs as more info becomes available - and of course, I'll be posting here.
My family is celebrating our 6th Christmas in 7 years this year - so I consider myself a bit of a veteran!