Someone's put up a cool instructional video (with helpful closeups of finger positions etc) of this great John Lennon song on YouTube.
You really have no excuse for not learning to play it, you know? (Unless your excuse is "I got so wasted at the Christmas party, I couldn't tell the fretboard from my ass". That's a perfectly valid excuse.)
Forget all that reindeer and Santa business. This is the real Christmas song. Simple chords, great lyrics, everyone loves it.