I went for thanksgiving 2018. It was noticeably more busy than my previous trips and rides that we've never had to wait long for-we had to bypass due to 30-40 minute waits for everything.
We arrived on the saturday before and crowds didn't pick up significantly until about tuesday. We went to MK the wednesday before and were surprised at how much we were still able to get down. Just make sure you use your fast passes to the best of your abilities and don't forget to keep adding more! We were pleasantly surprised that we were constantly able to add another fast pass for an attraction we were interested in (Haunted Mansion, Splash Mountain, we got the Ariel ride, we could've gone on Jungle Cruise with a FP but it was a late time and we wanted to make the fireworks, i forget what else i'm missing). I don't think every guest utilizes FP+ to the maximum ability so keep that in the back of your mind.
We use PH to add a little more flexibility into our stay. However, we probably wouldn't have the next time around if we go over a peak crowd time/holiday. For instance, we started at Epcot and went to DHS in the afternoon. We rope dropped soarin, had to wait 35 minutes for spaceship earth, the single rider line at TT was 50 minutes and we didn't even get to try for Frozen before we had made lunch reservations and fast passes for DHS.
Despite my ehhhh feelings towards more crowds you will still be able to get a lot done. I love that the christmas decorations/themed shows are displayed, the weather (at least for us) was practically perfect, and you'll be with your loved ones having an amazing time. Just go into it that you may need to take things slower due to longer lines, you may need to skip a few attractions, etc.