The Mayan calendar was not based on any solar or lunar event. It was a series of cycles within cycles within cycles. The calendar counts were based on a base 20 (we use decimal or base 10) system where each cycle was 20 times the previous cycle except for the second order which was 18 times. Thus the Mayan week was 20 named days long, the Winal cycle. The Winal cycle repeated 18 times (second order) the Tun which counted 360 days. After a Tun a 5 day period, a Wayeb, was added. These 5 days were considered unlucky. The Tun + Wayeb = 365 approximately a solar year. However it was not tied to seasonal events as it wandered a day every 4 years. There were many other cycles: a 9 Lords of the Night cycle; a 13 day religious cycle, a 584 Venus cycle, an 52 Winal + Wayeb (18,980 day) cycle the Calendar Round cycle, etc.
The third order cycle the Katun was 20 Tun or 7200 days or 20 solar years.
The fourth order cycle the Baktun was 20 Katun or 144,000 days or 394 solar years.
The completion of this cycle is the one which caused all the confusion on the 21st
The fifth order cycle has not been reached at this point in time. The Long Count cycles began at the Mayan creation date which modern experts place at Aug 11/12/13, 3114 BC (Gregorian). Counting forward December 21/22/23, 2012 was simply the day that the calendar went to the next Baktun, at Long Count 13.0.0.0.0.
Sandra Noble, executive director of the Mesoamerican research organization, stated that "for the ancient Maya, it was a huge celebration to make it to the end of a whole cycle". She considers the portrayal of December 2012 as a doomsday or cosmic-shift event to be "a complete fabrication and a chance for a lot of people to cash in. It would be similar to claiming that 99AD would end the world since our calendar is now out of dates. We, like the Mayans simply move into the next order of magnitude, or 1000AD. For those of you who wish to wait, the fifth order, the Piktun is 20 Baktun or 2,880,000 days or 7885 years reached at Long Count 1.0.0.0.0.0, will be on October 13, 4772.