This kind of stuff happens even outside poly romantic entanglements.
My BF and I haven't been together long enough to warrant spending holidays together, but DH and I deal with a similar rigmarole every Thanksgiving and Christmas - with two sets of divorced parents. That movie Four Christmases? Yeah, I can totally relate! The advice to do what YOU want is nice, but (IMO) its not realistic. Someone is always disappointed and/or offended. Ultimately, that is their problem, but we have to deal with the fallout, and want to avoid it if we can with some careful schedule finessing and loads of coffee
- Go see your parents on Xmas Eve
- Spend 2-4 hours first thing Xmas morning working
- Skip the extended family lunch and chill most of the day with GF while BF is with his family
- Have a late-ish dinner date and snuggles with BF in honor of your anniversary
You'll be totally exhausted, but everyone would know you value them by making sure you made time. I swear I'm tempted every year to say eff-it! and book a Xmas cruise.