Poul Anderson has space travelers celebrating Christmas light years from Earth and SM Stirling has time travelers celebrating it millennia in the past. Festivals mean what we make them mean.
Stirling's Nantucketers consume:
pumpkin pie sweetened with honey
no cream (milk too expensive)
After the dinner, a marriage is announced. A woman who was married in the twentieth century needs no divorce to remarry in 1250 B.C. Her first husband is not dead. He has not been born yet.