Please get your facts right.
1) Niburu was not a part of the Mayan cosmology. It was part of the Sumerian cosmology, who were nowhere near the Mayan's and were based in modern day Iraq.
2) Niburu is meant to be on an orbit that is eliptical to the plane, i.e. it's at a funny angle to the earth's orbit, unlike the other planets that are more or less on a flat plane in their orbits
3) The Mayan's worked on a 360 day calendar. The calculation on December 21st 2012 was based on the modern 365 day calendar, which means the date is about 80 years out.
4) There is apparently a major alignment of planets around 21st December this year where all the other planets are lined up on the opposite side of the sun.
5) The Mayans prophecy was the end of the 5th Sun, with a sun being a cycle of life on this planet. Many people see this change as being entering an age of enlightenment, the age of Aquarius, where people grow in their spirituality and get closer to the Source of life.