Picked up and confirmed by Android Police, it seems that Android 4.2's Contacts app simply omits December altogether.
Perhaps someone doesn't like Christmas. Perhaps it's an attempt to avoid the New Year hangover. Perhaps it's a last-ditch, desperate attempt to prevent the end of the world without using Bruce Willis.
Or perhaps it's just a bug.
It's already been submitted to Android's bug tracker, so hopefully we'll see a fix rather rapidly. We'd imagine that it'd be rather infuriating to receive a Christmas Android that swore the day it was opened didn't exist.