This is long, but there's a lot of info:
"First of all, the Mayans don’t have a calender they have calendars which often interlocked. The calender that has given rise to the myth of the end of the world is the Mayan long count calendar. According to Mayan Mythology, we are living in the fourth world or “creation” so to speak. The last creation ended on 12.19.19.17.19 of the long count calendar. That sequence will occur again on December 20, 2012. According to the Mayans this is a time of great celebration for having reached the end of a creation cycle. It does not mean the end of the world but the beginning of a new “age”. Does the world end every December 31st? No – we go on to a new year. This is the same as the Mayan creation periods. In fact, the Mayans make many references to dates that fall beyond 2012. The idea of 2012 being the end of the world was actually first suggested by New Age religionist José Argüelles in his 1987 book The Mayan Factor: Path Beyond Technology."
Then again,
"The Mayan Long Count, however, was invented, or so scholars believe, on August 11, 3114 BC, in Gregorian terms. It is based on the movement of the earth and sun, relative to the Milky Way galaxy. Thus it is extremely accurate. A very impressive feat, too. But according to it, we are currently living in the fourth age. And the numbers of the Long Count calendar are about to reset. They will do so on December 21, 2012, the winter solstice in the Northern Hemisphere.
The previous age ended on a long count of 12.19.19.17.19. Another 12.19.19.17.19 will occur on December 20, 2012, followed by the start of the fourteenth b’ak’tun, 13.0.0.0.0, on December 21, 2012. Conspiracy theorists are burning up the Internet claiming that this means the world will end. Their proof lies in the “Popol Vuh,” a Mayan book which compiles creation accounts known to the K’iche’ Maya people, and which states that this fourth age is the gods’ first successful attempt to create humans, after 3 failed attempts. Thus, the world as the Mayans viewed it, and as we live in it, is perfect, and there need not be a fifth age.
Given that they based their calendar on geometry involving the sun and the Milky Way, they must have seen some catastrophic end coming for Earth. This means an asteroid or meteor or comet, which is destined to strike Earth and wipe everything out on December 21, 2012. This is no immature claim. NASA is grossly outmatched by the night sky. All the foreign bodies floating around out there that have been sighted have been confirmed as no threat. But NASA calculates this is as a percentage so small, compared to the rest which we don’t know about, that the percentage is practically meaningless. It is 10 to an extremely negative exponent. There are many potential Earth impactors out there that no one has seen or knows about. Any one of them could have our name on it.
Or what about a massive solar flare? NASA expects a solar maximum from 2010 to 2012, a time during which the sun’s activity will be very busy. Flares are absolutely impossible to anticipate. They are bursts of light and heat, and thus cannot be seen before they arrive, because they travel at the speed of light. We will have no warning until we are walking outside and the sky explodes into brilliant light. And then Earth and everything on it and in it will be incinerated. Gamma radiation will penetrate to a mile in the crust and irradiate anything trying to hide.
There is also a theory amongst fundamentalist Christians which combines the Mayan Long Count with the “rapture”. According to the rapture theory (invented in the 1700s by protestant fundamentalist Cotton Matther who started the Salem witch trial hysteria), Christians are spared the Tribulation. But the Tribulation will last for 7 years, after half of which the Antichrist will rise to absolute power, and 3.5 years later, Jesus will return with all the dead who died in him, and the battle of Armageddon begins.
This means that if the Mayan Long Count is correct, then December 21, 2012 marks the beginning of the first or second half of the Tribulation. If it is the first, then anyone who believes he or she will be raptured up to heaven has a little more than 3 years left to pack a suitcase. If, however, December 21, 2012 marks the beginning of the second half of the Tribulation, when God wreaks all the really terrible stuff upon the earth, then those, who believe they will be raptured, will be raptured less than three months from the time this is posted."