It's not just a random date that Mayans decided to end their calendar on.
The calendar was based off celestial alignments and positions, which no one knows how an ancient civilization could know that without all of our telescopes and satellites and such.
On December 21st, 2012 the sun will be in the "exact" center of our galaxy, which would make an alignment with the earth to the sun and center of the galaxy.
Crackpot theorists say this could cause the poles to shift, which would completely screw up the earth...
Whether or not that is so, how could the Mayans predict such an alignment of the planets?