From some website:

The Long Count to the End
The Mayans have a complex system of calendars, and each calendar has a different purpose. There are thought to be 20 calendars in use, though only 15 have been revealed to the public. The Mayans keep the other five a secret within their culture. Some of the more commonly known Mayan calendars are:

The Tzolk'in Calendar - Used primarily in crop rotation, this calendar allows one 260-day period to ready the land and one 260-day period to grow and harvest the corn.
The Haab Calendar - This calendar lasts 360 days, with a 5-day period called the wayeb. Close to the Gregorian calendar we use today, this calendar follows the cycle of the sun.
The Calendar Round - The Calendar Round gave the Mayans a way to record history in longer increments. It's a combination of the Tzolk'in and Haab calendars and goes through about 52 years.
Fifty-two years was longer than the Mayans' average life span at the time of the Calendar Round's creation. However, Mayan historians wanted to create a calendar that could be used to record history for centuries. This led to the Long Count calendar. The Long Count incorporates an era called the Great Cycle, which lasts approximately 5,125.36 years [source: Jenkins]. The idea that the world is on its way to an end comes from the Long Count.

Sometime during the Spanish Inquisition, the Mayans stopped using the Long Count -- at least as far as the Spanish were concerned. A Mayan historian began recording events on both the Long Count and Gregorian calendars. Scholars then compared the dates on both calendars and confirmed the beginning of the current Great Cycle as August 13, 3114 B.C., making the end of the fourth Great Cycle -- you guessed it -- December 21, 2012. Theorists believe this is the day that the world will end, and all living things on Earth will die.

The end of the world may resemble another ice age.
However, the Mayans themselves don't actually believe that the world is going to end at the end of this cycle. In fact, they believe that it's a time of great celebration and luck when the planet lasts through a full Great Cycle. After all, we've made it safely through three other Great Cycles, and the world is still turning.

What makes this cycle so different, some believe, is that it ends on a winter solstice. On this particular winter solstice, the sun will align with the center of the Milky Way. This particular event happens only every 26,000 years or so [source: USA Today]. But is this celestial occurrence really going to end the world and kill us all? Probably not. Many scholars see this theory as extremists trying to cash in on the fears of others [source: USA Today].

So what will happen on Dec. 21, 2012? It's likely that the day will pass with no major event happening at all. People may not even realize it's the projected doomsday, although that's unlikely considering how much press it's likely to garner. We'll just have to wait and see what happens -- and hopefully update this article on Dec. 22, 2012.


Here's more on how people are cashing in on the 2012 hysteria
http://history.howstuffworks.com/central-american-history/world-end-in-20121.htm


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