For Christmas I usually make sure there's stuff around I can eat. I have a bit of a hand in the menu, no matter where we have our dinners. (At home, and with my family, and my inlaws, and with a friend. Christmas lasts forever!). I never leave the table hungry, and I don't leave the table high. Plenty of meat, fish and poultry, deviled eggs, cheeses, unsweetened whipped cream, that sort of thing. As for birthdays, I seem to have become the designated cake photographer with my inlaws. I photograph other people (read: the kids) demolishing theirs. Keeps my hands busy while I just have water and don't partake until snacks come into the picture. Usually cheeses and cold cuts here in the Netherlands, and considering that usually doesn't appear on the table until well after noon during a daytime party, that's my breakfast. For an evening get-together I might skip dinner and go with the snacks as my inlaws feel horrible if I can't join in. I don't care if I have to fast, but they certainly do. Vigorously. My husband and I celebrate our birthdays at an amusementpark we love as we both find parties stressful, and instead of cake I have a juicy steak, or salmon, or both!

I have more trouble with a different, Dutch holiday which mainly revolves around gifts for kids and lots and lots of candy for all. Of course, this past weekend we found a shop that sells the seasonal candy year-round rather than just in November/December, and my husband loves it as much as I do. I have 4 bags of different flavoured
kruidnootjes screaming at me from the pantry, because he forgot to take them to work with him. Not going to go there though.
All in all though, it's entirely up to you. Some people can have a cheat day here and there, just for the holidays or a birthday. All-out or moderately. I found I get carb cravings rather badly if I lapse, not to mention migraines, and hey, keto flu when I want to get back on the wagon! So I stay on it for the most part. But for you it might be entirely workable to say, share a slice of cake with someone, or have some and have a quick walk after, or...? What works for one, may not work for another. You'll find out as you go along.