The last year cycle of the Mayan calendar does indeed end on December 21, 2012, but the reason for this is less mysterious than one might think. First we have to understand how the Mayans kept time, which is largely more complex than our own. They two systems, but the one of importance is known as the "Long Count", which dates using five values:
Kin = 1 day
Uinal = 20 days
Tun = 360 days
Katun = 7200 days
Baktun = 144000 days
A Mayan date like 6.3.3.8.0 would be 6 baktuns, 3 katuns, 3 tuns, 8 uinals and 0 days. A full Mayan cycle is 13 baktuns, which means it would end on 13.0.0.0.0, which would be 1,872,000 days from the initial date of 0.0.0.0.0, or more commonly known as the "Zero Date". Through some crafty archeological work, most experts agree that the Zero Date is August 11th, 3114 C.E. on the Gregorian Calendar.
What's interesting, however, is that the Zero Date was initially of little relevance to the Mayans because their calendar system was based on its end rather than its beginning, meaning that the 2012 date was chosen first, then the cycle was retroactively applied to give the current date.
The Mayans and other Mesoamericans were concerned on many levels the concept of emptiness, death, etc., which may help explain why they were the first to develop the number 0 and have still the only calendar system that incorporates it. Their astronomical observations are often based on the voids between star movements, like the the northern void where Polaris now resides today. 2012 marks the end of another age, but why was this date chosen in particular, if it is to be assumed at all?
The most conventional explanation is that on this date the winter solstice sun aligns through the "dark rift" in the Milky Way galaxy, a rare phenomenon. Why this phenomenon is significant has its origins in Mayan mythology, namely the Sacred Tree, an image heavily referenced in their culture. Astronomically, the Sacred Tree is represented by the intersection of the band of the Milky Way galaxy with the ecliptic of the sun. This intersection is considered the doorway between life and death, among other things.
It must be assumed that the Maya were able to predict the precession of the equinoxes, which is becoming increasingly more accepted amongst archeologists. It's not too surprising, seeing as their calender can be used to predict just about everything else. The relationship between the ecliptic and the Galactic Equator is one heavily referenced in Mayan mythology, so it's not much of a stretch to assume that the Maya would set the end of their calendar to correspond with what they viewed as a significant celestial event.
The band of the sun passing through the dark region in the Milky Way represents a passing through the void, resetting the relationship between the worlds of the living and the dead. What most conspiracy and apocalyptic types fail to understand, however, is that this event doesn't hail the end of the world so much as it is the resetting of the cosmos so that a new age may begin.
All of the above, naturally, is a complete lie.
