Ian O'Neill of Universe Today explains why the Mayan calender ends on December 21, 2012:
The base year for the Mayan Long Count starts at "0.0.0.0.0". Each zero goes from 0-19 and each represent a tally of Mayan days. So, for example, the first day in the Long Count is denoted as 0.0.0.0.1. On the 19th day we'll have 0.0.0.0.19, on the 20th day it goes up one level and we'll have 0.0.0.1.0. This count continues until 0.0.1.0.0 (about one year), 0.1.0.0.0 (about 20 years) and 220.127.116.11.0 (about 400 years). Therefore, if I pick an arbitrary date of 18.104.22.168.1, this represents the Mayan date of approximately 1012 years, 7 months and 1 day.Seems like the ancient Mayans were quite clever, but not quite clever enough when it came to calendars. But then, their civilization ended long before their calender did, so I guess it worked out for them.
This is all very interesting, but what has this got to do with the end of the world? The Mayan Prophecy is wholly based on the assumption that something bad is going to happen when the Mayan Long Count calendar runs out. Experts are divided as to when the Long Count ends, but as the Maya used the numbers of 13 and 20 at the root of their numerical systems, the last day could occur on 22.214.171.124.0. When does this happen? Well, 126.96.36.199.0 represents 5126 years and the Long Count started on 0.0.0.0.0, which corresponds to the modern date of August 11th 3114 BC. Have you seen the problem yet? The Mayan Long Count ends 5126 years later on December 21st, 2012.