You may have seen this image floating around Facebook lately:
You may even have shared it yourself—and that's okay. There are so many things wrong with the "2012 Mayan Apocalypse" (the belief that the world will end at the conclusion of the thirteenth b'ak'tun of the ancient Mayan calendar) that it can be tempting to just pile on whenever the subject comes up.
But I'm afraid I'm going to have to burst this particular doomsday bubble: the Mesoamerican Long Count calendar is not a solar calendar, so the idea of a leap day makes absolutely no sense.
The Long Count calendar, which (supposedly) ends in December (except that it doesn't) is basically just a tally of the number of days that have elapsed since 11 August 3114 BCE (the creation-date of the universe*).
While it seems that their solar calendar would have required occasional adjustment if they wanted to keep in in line with the seasons, it's unclear what a leap year would even mean in the context of the Long Count calendar—which is what we're talking about in the context of 2012.
This image (and those like it) is guilty of speaking of a single "Mayan calendar", when the ancient Maya used several (at least three) different calendars for different purposes. Let me explain.
When someone says "calendar" these days, he or she is typically referring to the modern Gregorian calendar, which is a solar calendar. Solar calendars use leap days (according to certain rules, with varying degrees of complexity) to attempt to keep the equinoxes (and thus the seasons) aligned to certain calendar dates (with varying degrees of success). This is necessary because a calendar year contains 365 days, while a solar year contains an average of 365.24219 solar days (the length of the year and the day having little to nothing to do with each other).
A solar calendar is not your only option, however. Lunar calendars add additional complications, because the rate at which the Moon orbits the Earth is not directly related to the rate at which the Earth orbits the Sun. If a lunar calendar attempts to keep the seasons aligned to certain calendar dates (a lunisolar calendar), then leap months (intercalcations) are inserted from time to time (instead of leap days) to keep everything in order.
So which one of these is Mesoamerican Long Count calendar? Well, as I said before, it's not any of those. There were no years or months in the Long Count: just a number of days that counted up and up and up in a (slightly modified) base 20 system. The third "digit" of the calendar corresponded to a period of 360 days, but we know that the Mayan year was 365 days. The ancient Maya did use a solar calendar, called the Haab', which did not employ a leap year, so in that sense the image above is correct—but the Haab' has nothing to do with the purported apocalypse.
The point is, everyone who has ever attempted to convert a date from the Long Count calendar into our own Gregorian calendar would be aware that the Long Count doesn't use leap years—because the whole concept of a leap year doesn't make any sense in a calendar that doesn't use years to begin with! This means that what the image is actually saying is that everyone who has ever attempted to convert a date from the Long Count calendar into the Gregorian calendar somehow managed to forget that we use leap years!
Don't get me wrong: there's absolutely no reason to think that the world will end on the 20th (or the 21st) of December (or even that the ancient Maya predicted that it would!). But, contrary to the claim being made here, everything that I've been able to find indicates that 20 December 2012 is indeed the last day of the thirteenth b'ak'tun. I do know that the Long Count is set to roll over to the fourteenth b'ak'tun on our modern Gregorian date of 21 December 2012 (a fact confirmed by the New York Times, incidentally).
There are plenty of good reasons to believe that the supposed 2012 apocalypse is a load of fetid dingo's kidneys; there's no need to go invoking bad ones.
* Of course, you and I both know that the universe wasn't created on 11 August 3114 BCE: it actually came into existence on 23 October 4004 BCE!