A fallacy based on bad astronomy. Here's what NASA says on the subject:
http://www.nasa.gov/topics/earth/fea...012-guest.html
Here's what I consider the most pertinent part of that article:
"Fast-forward to 1995. That year John Major Jenkins packaged several of these themes into Maya Cosmogenesis 2012. According to Jenkins, the winter-solstice point and the centerline of the Galaxy will line up exactly on Dec. 21. Arguing that this motivated the Maya to contrive the calendar to end on that date, Jenkins concludes that it will be "a tremendous transformation and opportunity for spiritual growth, a transition from one world age to another."
In fact, astronomy cannot pinpoint such a "galactic alignment" to within a year, much less a day. The alignment depends on the rather arbitrary modern definition of the galactic equator, and/or the visual appearance of the Milky Way. There is no precise definition of the Milky Way's edges -- they are very vague and depend on the clarity of your view. (Jenkins says that he personally established the Milky Way’s edges by viewing it from 11,000 feet, far above anywhere the Maya lived.) So to give a precise visual position for its centerline is not meaningful.
Jenkins did acknowledge that the winter-solstice Sun actually crosses the center of the Milky Way anytime between 1980 and 2016. Elsewhere he expands this approach zone to a 900-year period, and settles for an imprecise alignment to which Dec. 21, 2012, is arbitrarily and circularly assigned. Real astronomy does not support any match between the Baktun-13 end date and a galactic alignment. The advocates both admit and ignore this discrepancy."