December Returns to Android People App With 4.2.1 Update
The problem affected those who upgraded to version 4.2 of Android Jelly Bean. Users who tried to enter in a December birthday or anniversary in the People app were out of luck; it skipped right from November to January. It did not affect the Calendar app.
Nexus Android 4.2.1 update rollout, no changelog yet
Android 4.2.1 update brings December back to the People
Google Rolls Out Android 4.2.1 OTA Update, Fixes December Bug