You will only gain weight if you overeat or if you eat a diet high in sugars or alcohol. And the awesome news is that you can enjoy the special events and not care because you have a diet that will help you lose the weight after. Just enjoy life. As Dr Fung says, ”When you fast, fast. When you feast, feast!”
I am on week 3 of a keto break. I still try to fast at least 12 hours and usually get my 16, and eat as keto as I can. I have had very minimal weight gain (maybe about 5 lbs?), but I know that some fasting will melt it all away at the end! You will always be able to lose any gained weight with the keto/IF method! Tbh, I’ve noticed that the fasting is what really makes a difference.
If you do keto/IF for months and lost 100 pounds, then stop for a week, then no, you won’t gain 100 pounds. You might however gain 5 … 10 pounds.
Also, if you just started keto, you can lose 15 pounds in a week. If you then stop keto, you can gain that back rapidly.
Some of the weight loss is water weight, that’s what goes immediately and rapidly. Also what can return quickly.
Overall actually body fat, goes slowly and would come back slowly.