December 21st, 2012 is when the Long Count (the Mayan calendar) reaches its last b'ak'tun. This is no cause for alarm if ONE civilizations' calendar says the world is going to end then. The Mayan calendar also predicts events past the date of 2012. In their culture, this is how the Mayans thought the world would end: a figure named "First Father" would be challenged by the Devil, and play a game very similar to soccer, where you had to hit the ball through a small hole on a wall. Through trickery and deception, the Devil defeated First Father. First Father's 2 sons would then challenge the Devil to a game of this soccer-like sport, and they would defeat him, freeing first father. The Devil would then open a portal to the underworld (black hole, much?) in his last attempt to defeat First Father and his sons. The world was supposed to end over a hundred times, and it hasn't. Although it has seen an enormous deal of strife, it's still standing.
From a Religious point of view, Nostradamus predicted the world would also end on December 21st, 2012. So did Merlin, the prophet from the stories of King Arthur (he was real, but a much darker 'sorcerer' in actuality). Then there was Mother Shiptin, and this date even appears in Chinese culture. However, so did Y2K, 2007, WWII, and many other events. These things did not end the world.
From an astrological point of view, the Sun is supposed to be aligned with the middle of the universe, where a supposed black hole is supposed to be. Or, the earth's polar structure is supposed to change, causing North to be South, and South to be North. Then there's the Planet X theory, which I'm not even going to converse about because it's so completely illogical.
My personal view is we have about a 100% chance of happening. Because the world can end any second. December 21st, 2012 should be no different.