As part of my RSS reading, Smashing Magazine is rapidly becoming something I look forward to reading. Every month they do a roundup of desktop wallpapers which include a calendar and are, in some way, seasonal. This month is no exception! They have a fantastic selection of christmassy wallpapers for you to chose from and they're all free!
Checkout the wallpapers and, while you're at it, checkout the rest of the site!
http://www.smashingmagazine.com/2008/11/30/desktop-wallpaper-calendar-december-2008/