Google has promised to bring 'December' back to Android 4.2 soon. Earlier this week, reports emerged how the People app in the latest version of Android (v4.2) does not let you set any date in December as the birth date of a contact (see screenshots above).
The issue was first reported to the Android development mailing list on November 14. Android developers acknowledged the bug in the developer mailing list and now Google has taken to Google+ to promise that a fix for the problem is coming 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.