In this video from the Discovery Channel, a scenario is shown when an asteroid falling into the Pacific Ocean is 500 km in diameter. I do not want to frighten anyone, however, it is argued that this has happened to our planet at least 6 times before!
(I highly recommend to enable HD mode in the player window, and it is better to switch to YouTube itself for viewing on a large screen!)
In fact, it’s not the calendar that ends, but the so-called Great Cycle. Or "The Fifth Sun" in Maya terminology with a length of 5126 years. The last day of this cycle is December 21, 2012. Then the next one will begin. So do not worry! :)