Whilst the internet has been bombarded with stories on December 21st, the day could mark the time where Nintendo will reveal The Legend of Zelda: Majora's Mask for Nintendo 3DS.
According to an anonymous source on Zelda Informer, earlier today Nintendo of America supposedly tweeted "Hey Zelda fans! On the dawn of the final day, a familiar game will be announced for the Nintendo 3DS! Look for it this Friday!"

Of course as with all rumours, the unconfirmed tweet has since disappeared and the language seems a tad different from the account's most recent Tweets.
Whilst an announcement on December 21st does seem unlikely, the prospect of a Majora's Mask for Nintendo 3DS is a conceivable on - given the quick and successful turnaround time for Ocarina of Time 3D last year.