Google has acknowledged the bug in a post to the Android Google+ feed. The Android maker promised to release a fix soon:
We discovered a bug in the Android 4.2 update, which makes it impossible to enter December events in optional fields of the People app (this bug did not affect Calendar). Rest assured, this will be fixed soon so that those of you with December birthdays and anniversaries won’t be forgotten by your friends and family.It's surprising that a bug of this scope could make it out into the wild, but it's not unheard of. Things can be missed by even the most careful QA testing.
Still, the question remains: how soon is soon. Google could choose to delay a release until after Dec. 21. After all, if the Mayan Apocalypse comes to pass, why bother with a new Android release?