There is much discussion on Paleo forums about how to deal with events such as birthdays, holidays, office parties and other times when non-Paleo foods will be served. Some say to eat Paleo foods before going and not eat at the event, or eat only what is safely Paleo. Others say to fast during the event. Still others take a more moderate approach of avoiding only the worst non-Paleo foods such as wheat and desserts.
Celebrations are important for tribal bonds. Paleolithic hunters and gatherers would have considered tribal bonds more important than their own personal needs. If a group of your dear friends go out for beer and pizza once in a while, I say go. Eat one slice of pizza unless it is going to make you seriously ill. If your mother or someone else traditionally makes a certain dish for Thanksgiving, have a few bites of it unless, again, it will make you sick. Your health is important.
It doesn't just depend on what you eat though. It also depends on your tribal bonds. If you don't have any tribal bonds, I would suggest that you get some. Mark gave a wonderful reply to someone who asked how to mostly stay Primal when doing field work in Nicaragua ". . . when a campesino offers you a plate of hard-won beans grown by the family, it doesn't sit well to turn your nose up at it and talk about carbs!"
At a buffet dinner for work? Sure, go ahead and politely pass. . . . But when a host family offers the clothes off their backs or the beans from their pantry, you generally accept. Definitely make those concessions when they're necessary – provided you don't have serious digestive issues with particular foods that you know will cause problems.
Yes, ideally your friends and relatives should be respectful about your needs. But it isn't all about you.