Good advice so far. The one thing I like to do is have bone broth at the ready to break my fast when I want, even if not at my goal. I usually have bone broth, wait an hour or two and then eat a real meal (not too big, but lean protein like Salmon). Have it ready for when you decide that is it so you are not feeling bad while trying to get it ready as well.
Not a good idea after christmas tbh, youll have so much bad stuff in your system it’ll make succeeding insanely hard.
How about you go keto a week after Christmas and then begin your fast? Much less doomed to fail! Happy holidays, definitely make sure youre enjoying xmas☺️