I'd find a bunker where the 2012 nutjobs are hiding. I'd run in covered with fake blood, screaming "RAINS OF BLOOD! THE APOCALYPSE! REPENT THY SINS!"
I'd need an accomplice with a video camera, because their reactions would be priceless.
More likely scenario: I say "Know what today is?" to my friends, do some Christmas shopping, remind & mock a friend about the time he messed up his calendar and thought he had been in a coma for a month. Also, many video games, because on December 21st I'll have just started my Christmas vacation.
Fallout 4 and Left 4 Dead 3 will hopefully be in the Xbox 360 that evening.