I happened to read some great advice on fetlife today about this subject. The person said
"Discuss holiday traditions at least 3 months before the holiday. Just fondly reminisce and ask your partner/s to do the same.
Now you'll all have a good idea of what might conflict before the emotion laden dates arrives"
I know it's a little late for this holiday, but I think it's a great for many things (going to a party for the first time when one of you will have two partners and you aren't sure how to act, addressing PDAs, and many other things that if you wait til last minute to talk about, can easily lead to emotional responses or confusion about what's going on).
Happiness will never come to those who fail to appreciate what they already have.