I already had my playlist ready since September.
I'd definately include:
Happy Xmas (War is Over) - John Lennon
Jingle Bell Rock - The Muppets
12 Days of Christmas - The Muppets
Little Drummer Boy - David Bowie and Bing Crosby
Santa Baby - Marilyn Monroe
Nat King Cole's version of The Christmas Song
Baby, It's Cold Outside - Margaret Whiting and Johnny Mercer -> My favourite, not xmas per se, but it is a winter song and I've always related it to the holidays