If your birthday, anniversary, or another special day falls in December, and your handset is running the latest version of Android, 4.2 Jelly Bean, you'll find that the month of December is missing in the People/Contacts app. This app comes as a stock application, and is a part of the Android Open Source Project (AOSP).
reports that in the People/ Contacts app, Google has forgotten to include December. Google has added only 11 months in the calendar year. After November 2012, comes January 2013.
Android Police has reported the issue, and Google has acknowledged it. Android Police states, "After digging through the AOSP source, I think the bug can be likely found in one of these commits, probably in the packages/apps/Contacts/src/com/android/contacts/datepicker/DatePicker.java file
This is a rather big problem and from the looks of it, Google should release an update with a fix in the near future. Google announced Android 4.2 last month, and this version still retains the Jelly Bean branding like Android 4.1.