We’re departing Disney on 20 December. Split stay. So my experience is not helpful likely since you’re interested in Christmas day particularly.
I had overthought dinners because most of our favorite restaurants are closed. Finally decided on Kona for our arrival supper. I didn’t get on line until about 8 am central time and 5 pm was still available.
As we’re at AK on Friday, I decided to do any Yak and Yeti available when our next ADR window opened. But I completely forgot yesterday that it was ADR day. I didn’t recall at all until just now reading your post. A quick search shows that I’m now going to be doing the reservation search for that Yak and Yeti meal any time on Friday 18 December.
So definitely log on by 5 am our time!