With the rollout of a major OS upgrade like Android 4.2, we can imagine it's easy to forget something, but Google's omission of the entire month of December in the People app was certainly a doozy. The beauty of the Nexus program is that Google can rectify little issues like that post-haste, however, and Mountain View has now rolled out Jelly Bean 4.2.1, a small update that adds back that all-too-important month. So far, Nexus 4 and 10 users have reported getting the update, according to XDA Developers, though there's no evidence yet of any other fixes. Check your Nexus device (or the source) to see if it's arrived in time for, you know.
Update: Looks as if the Nexus 7 is a go, too!