Here are some great Christmas themed RPGs to get your group in the holiday mood:
Spirit of the Season – Contains rules for holiday one-shots using either theSpirit of the Century (FATE) or Truth & Justice (PDQ) rule sets. This supplement is a lot of fun and well worth the small price.
It Happened One Christmas – A free adventure for the excellent Faery’s Tale RPG.
The Secret Lives of Gingerbread Men – A family RPG in which you bake and decorate cookies which then act as your character sheets. This is on my “to play” list this Christmas.