Android 4.2.1 brings back December, rolling out to Nexus devices now
The small bug on Android Jelly Bean meant that the 12th month was missing from the list, meaning you couldn't, for example, add a birthday in December to a contact in the People app. Google hasn't listed any other changes that come with the update.
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Google save Christmas! Android 4.2.1 Jelly Bean update brings back December