YMMV, but we’ve been during the same time frame for the past two years. For us, it hasn’t seemed overly crowded at all, but we make it habit do all the usual crowd avoiding tricks — get into the parks early, take long midday breaks, spend some time resort hopping, enjoy restaurants at resorts instead of parks, etc. That suits our natural preferences but probably also makes a difference in how crowded it “feels” to us. In my experience, it starts to feel more unavoidably crowded around Dec 23 or so. In full disclosure, we’re a slave to the school calendar so we travel almost exclusively during high crowd times which probably makes our definition of “crowded” different than other folks.
Our anniversary is Dec 21! Hope you have an amazing trip!