It's a very specific date - where did it come from? December 21 will be the (northern hemisphere) winter solstice of 2012. Sounds significant, right?
But how did this come from the Maya, who had a completely different calendar system? The Maya calendar is translated to our current (Gregorian, to use its proper name) calendar using a system called "GMT calendar correlation". This is based on comparing recorded events from Mayan times between the 2 calendars. It sounds fairly straightforward, but a lot of people who know a LOT more about Maya history than you or i have been, or still are debating its accuracy.
According to some, this GMT correlation is totally 100% unquestionably correct. According to others, it may be as far as 60 days, or even 104 years out. The problem i have here is that many of the folks defending the GMT correlation have a personal or professional interest in maintaining its credibility. If it were to be disproved and modified, Maya history would have to be entirely re-dated. Also, the whole December 21, 2012 thing would lose a little of its appeal if it were to be suddenly changed to: DOOMSDAY: August 15th, 1982, or DOOMSDAY: SOMETIME AROUND LATE 2012 OR EARLY 2013.
So there you go, December 21st: not even totally reliable. Having a precise date does help sell the various theories though, particularly the Galactic Solstice Alignment theory, which i'll go into in the next post.