The day the Mayan calendar ends, but not exactly the end of the world. The Mayan calendar is just like our standard calendar only they created a longer one, most likely their calendar was ment to repeat... not just end. So don't worry the world will be around for many thousands of years.
Person: OH SHIT WE GON DIE DECEMBER 21, 2012
Dude: You are a fucktard TODAY IS THE 22 that means it didn't happen
Person: OHMYGOD NO CHRISTMAS!
Dude: You are retarded I just said it didn't happen which means #WESTILLHAVECHRISTMAS
Person: oh...
Dude: What a stupid person.