The Mesoamerican Calendar was a method of tracking time used with some variations by the Aztecs, Zapotecs, Maya; in fact all of the Mesoamerican societies when the Spanish conquistador Hernan Cortes arrived in AD 1519. This shared calendar had two parts that worked together to make a 52-year cycle, during which each day had a unique name. One cycle lasted 260 days, and the other 365 days.
The two calendars together were used to keep chronologies and king lists, mark historical events, date legends, and define the beginning of the world. The dates were chiseled into stone steles to mark events, painted on tomb walls, carved onto stone sarcophagi and written into bark cloth paper books.
The Sacred Round
The 260-day calendar was called the Sacred Round, the Ritual Calendar or the Sacred Almanac, tonalpohualli to the Aztecs and piye to the Zapotecs. Each day in this cycle was named using a number from one to 13, matched with 20 day names. The day names varied from society to society. Scholars are divided about whether the 260 day cycle represents the human gestation period, or some astronomical cycle, or the combination of sacred numbers of 13 (the number of levels in heaven, according to Mesoamerican cosmogeny) and 20 (Mesoamericans used a base 20 counting system).
The most famous representation of the sacred round is the Aztec Calendar Stone. The twenty day names are illustrated as pictures around the outside ring. The oldest use of the sacred calendar found to date was on a threshold stone from the site of Zapotec site of San Jose Mogote. The stone, roughly dated ca 600 BC, has a day name from the sacred round carved on it. Marcus and Flannery have argued that by first century AD new temples were built at San Jose Mogote every 52 years. The sacred round is still used today in some Maya communities.
Because the sacred round was used by essentially all Mesoamerican communities, it is considered the oldest calendrical method used there.
Each day in the sacred round had a particular fate, and, like in western astrology, an individual's fortune could be divined on the basis of her birth date. Wars, marriages, planting crops, all were planned based on the best days for starting out.
The Solar Round
The 365-day solar round, the other half of the Mesoamerican calendar, was also known as the solar calendar, haab or tun to the Maya, xiuitl to the Aztec, and yza to the Zapotec. It was based on 18 named months, each 20 days long, with a five day period to make a total 365. The Maya, among others, thought those five days were unlucky.
Of course, the earth's rotation is 365 days, 5 hours and 48 minutes, not 365 days, so a 365 day calendar throws an error of a day every four years or so. The first human civilization to figure out how to correct that was the Ptolemies in 238 BC, who in the Decree of Canopus required that an extra day be added to the calendar every four years; such a correction was not used by the Mesoamerican societies.
The earliest representation of the 365 day calendar dates about 400 BC.
The Mesoamerican Calendar
Combining the Solar Round and Sacred Round calendars provides a unique name to a each day in a block of every 52 years, or 18,980 days. Each day in a 52 year cycle would have a day name and number from the sacred calendar, and a month name and number from the solar calendar. The combined calendar was called tzoltin by the Maya, eedzina by the Mixtec and xiuhmolpilli by the Aztec. The end of the 52-year-cycle was a time of great foreboding that the world would end, just as the end of modern centuries are celebrated in the same way.
The Maya Long Count added another wrinkle to the Mesoamerican calendar; but that's another story.
Clark JE, and A Colman. 2008. Time Reckoning and Memorials in Mesoamerica. Cambridge Archaeological Journal 18(1):93–99.
Gasco, Janine. 2001. Calendrics. pp. 90-92 in The Archaeology of Ancient Mexico and Central America: An Encyclopedia Edited by Susan Toby Evans and David L. Webster. Garland Publishing Inc., New York
Marcus J. 2006. Mesoamerica: Scripts. p 16-27 in: Brown K, editor. Encyclopedia of Language & Linguistics. Second ed. London: Elsevier.
Marcus, Joyce and Kent V. Flannery. 2004. The coevolution of ritual and society: New 14C dates from ancient Mexico. Proceedings of the National Academy of Sciences 101(52):18257-18261. Free download.