12-21-2012 Marks End of The 13th Baktun per Mayan Calendar
The first thing to understand is that the Maya used three different calendars. The first was the sacred calendar, or Tzolk'in, which lasted 260 days and then started over again, just as our 365-day calendar refreshes once it hits Dec. 31. This calendar was important for scheduling religious ceremonies.
The second calendar was the Haab', or secular calendar, which lasted 365 days but did not account for the extra quarter-day it takes the Earth to revolve around the sun. That means the calendar wandered a bit in relation to the seasons.
The final calendar was the Long Count Calendar, the recording method that has caused all of the doomsday brouhaha of 2012.
On Dec. 21 2012 (approximately), the Mayan calendar completes a major cycle marking end of the 13th Ba'k'tun, which has triggered doomsday fears and mystical rumors about the end of an age.