What is your character doing for Christmas? (Or Hanukkah, or Ramadan, or their birthday, or their S.O.'s birthday, or their anniversary, or Wookiee Life Day, or the Prince's Embrace night?)
I've prattled on at some length about GMing Christmas Specials before, but this can be a more general question for any time of celebration.
Buying presents can be an adventure in and of itself. (I say this as someone who managed to get both my parents books they already had this year. In my defence, my mother had dropped hints about Sir Terry’s Snuff and then went and bought it herself after I did...)
Then consider all the tradition and symbolism of such events. What do you do, what do you eat, where do you go, what games do you play, what do you wear, where did that whole mistletoe business come from?
And how do PCs from Culture A deal with the traditions of Culture B? What’s a good gift for the Ambassador of Barsoom?