They do sell out and quickly. There will be restaurants that will openings, but it might not be a place you want. The popular restaurants sell out fast.
Germany and Morocco seem to always have openings up until December. LOL And it may not be a day/time you want.
So I suggest to get on there, if not at 6 am the day of, at least sometime that day to get what you want.
If you're online at 6 am, same chances as everybody else.
Good to know, since I have never been at Christmas before, I have never paid attention. As a follow up, how far in advance do they typically announce when they will start taking reservations?