Christmas Day PSs are very limited already. I called to change an existing booking from the Yachtsmans Steakhouse to Artists Point and the only time left was 4.10pm!! This was too early, so got one of the last remaining PSs for the California Grill (7.10pm). I got the CM to do a quick search at other places (we have small children so wanted to eat around 6pm) and there was very little available at the popular places. Ohana had some slots as did Boma (not too Christmassy I guess) but I would bet these wont be around for too long. Book as soon as you can and be ready with a choice of places (and times) you would be happy with to avoid disappointment.
Good luck!!