Looks like 2013 is coming early for 4.2 users.
The latest version of Android's Jelly Bean comes with an interesting bug. Apparently, users are unable to add birthdays for contacts born in December because the month is missing. The issue was first reported by a user posting to the Android bug tracker on November 14. This user says he noticed the bug when trying to add a birthday to a contact for syncing with the Calendar app.
"Today I was inserting birthday of a friend in my contact list so that I can sync the birthdays in my calendar. I noticed that the December month is missing," he wrote. "When I pressed new event the calendar came but it was not having the option for the month December. I'm using Galaxy nexus running on 4.2 jelly bean which I installed yesterday on 14 November 2012. Please help me with this as I use the calendar a lot Please help me with this as I use the calendar a lot."
Google has acknowledged the error but has yet to offer a timeframe for a fix. For now, looks like you'll have to go back to marking birthdays on a real calendar. Oh, and don't forget December 25 is Christmas day.