What Was the Aztec Calendar and How Did It Work?

Unearthed in 1790, the Aztec Calendar Stone is one of the most impressive artifacts recovered from the advanced civilization that ruled central Mexico for centuries, but it’s not actually a calendar. Also known as the Piedra del Sol, or Sun Stone, the monumental basalt disk is etched with intricate reliefs, including the 20 named days of the Aztec ritual calendar, which led early interpreters to believe it functioned as a calendar.

“No Aztec person would say, I have to go look at the Calendar Stone to see what day it is,” says Michael E. Smith, professor emeritus of archaeology at Arizona State University. Ceremonial objects like the Calendar Stone were carved for each Aztec emperor and were meant to project the divine authority of the empire and its geographic dominance.

Smith thinks that the inclusion of day names on the Calendar Stone was “probably to show that the Aztec empire was timeless and controlled the past and the present and the future. It’s a big propaganda device.”

While the Calendar Stone itself wasn’t a timekeeping tool, it hints at the centrality of calendars to ancient Mesoamericans, including the Maya and the Aztecs, who called themselves the Mexica. The Aztecs carefully tracked the passage of time using three interconnected calendars: a 260-day ritual calendar, a 365-day solar calendar and a 52-year Calendar Round that, according to Aztec prophecy, would one day end with the destruction of the world.

The 260-day ritual calendar (tonalpohualli in Nahuatl) is the oldest timekeeping system in Mesoamerica, used by cultures including the Aztecs, Maya and Olmecs for at least the past 3,000 years. Unlike most ancient calendars, its length doesn’t align with either solar or lunar cycles; no one knows for sure what led to this departure from convention. Whatever its origins, the Aztec 260-day ritual calendar was mostly used by priests and professional diviners to determine auspicious and inauspicious dates throughout the year.

The ritual calendar is composed of two interlocking cycles: a list of 20 named days (or day signs) and a count of 13 numbers. Here are the 20 named days in Nahuatl with their English translations:

On the 260-day ritual calendar, the 20 named days are also assigned a number between 1 and 13. So the first day of the ritual calendar is 1 Cipactli, followed by 2 Ehecatl, 3 Calli, 4 Cuetzpalin, etc. When the day count reaches 13 Acatl, the numbers start over again, but the named days continue to the end of their cycle. So the next day after 13 Acatl is 1 Ocelotl, followed by 2 Quauhtli, 3 Cozcaquauhtli, 4 Ollin, etc.

The Aztec ritual calendar has 260 days because it takes 260 unique combinations of 13 numbers and 20 named days for the calendar to start over again on 1 Cipactli. For the Aztecs, the day on which a person was born also functioned as a “secondary name,” says Smith, author of At Home With the Aztecs: An Archaeologist Uncovers Their Daily Life. “You might be known as ‘6 Dog’ or ‘7 House.’”

For timekeeping purposes, the Aztec ritual “week” was 13 days long, a period the Spanish called a trecena. There were 20 trecenas in the Aztec ritual year, and each trecena was known by the day name assigned the number 1 for that cycle (so the first trecena is Cipactli). There was also a specific god that presided over each trecena, which could be a good or bad thing. Priests and diviners consulted illustrated charts of the 260-day calendar to determine which dates were lucky or unlucky for certain activities.

“If you’re a merchant about to set out on a journey, you’d go to your local priest and ask, ‘When should we leave?’” Smith says. “He’d pull out one of these codices and say, ‘This is a bad trecena. There will be floods and other disasters. But if you wait seven days, the next trecena is associated with prosperity, rainfall and fertility.’”

It was particularly important for the Aztecs to get married on an auspicious date, so arranging the timing of a successful marriage was one of the specialties of matchmakers and soothsayers.

As careful watchers of the sky, the Aztecs also knew that a solar year was 365 days, so they had a separate calendar used to track the changing seasons and celebrate communal festivals.

The 365-day solar calendar (xiuhpohualli in Nahuatl) was divided into 18 months, each composed of 20 days. That totaled 360 days, with the remaining five days added at the end of the year. Those five days were famously unlucky. (Because a true solar year is 365.25 days, the Aztecs may have also thrown in a leap year every so often to keep the seasons aligned, but it’s not clear, says Smith.) When early Spanish sources described the Aztecs’ 365-day solar calendar, they focused on the 18 different ceremonies and celebrations that corresponded to each month.

“These monthly ceremonies were a big deal, and they would last for days,” Smith says. “There would be priests doing sacrifices on a pyramid. There’d be processions through the streets. The young woman would do a ‘popcorn dance’ where they put strings of popcorn around their neck. And in their houses, people would offer incense and other gifts to the gods.”

One of most significant numbers in the Aztec world was 52. That’s the number of years it took for the two calendars—the 260-day ritual calendar and the 365-day solar calendar—to cycle through all the possible date combinations and restart on the same day. That’s because the least common multiple of 260 and 365 is 18,980 days, or 52 years.

Each completion of the 52-year cycle is known as a Calendar Round. The Aztecs kept track of what year it was within each 52-year Calendar Round using a system similar to the ritual calendar. There were four named years or “year bearers”: Calli (House), Tochtli (Rabbit), Acatl (Reed) and Tecpatl (Flint). Each named year was assigned a number between 1 and 13. The first year in the Calendar Round was 1 Calli followed by 2 Tochtli, 3 Acatl, 4 Tecpatl, 5 Calli, 6, Tochtli, 7 Acatl, etc. The 13 numbers multiplied by the four named years also equals 52.

The only problem with a 52-year cycle is that there’s no way to track the longer passage of time. Every combination of a number and year name repeats itself every 52 years, so it’s hard to know exactly when a historic event occurred. The founding of the Aztec capital Tenochtitlan, for example, was recorded as the year 2 Calli, but 2 Calli could have been the years 1273, 1325, 1377, 1429, 1481 or many others. Historians have determined that it was 1325 by cross-referencing with other dated events.

The 52-year Calendar Round is also significant because it’s linked in Aztec mythology with the end of the world.

According to Aztec myth, the gods created and destroyed the world four times. Humans are now living in the fifth creation or the Fifth Sun. No one knows exactly when the fifth sun will be extinguished, but Aztec belief held that it would happen at the end of a 52-year Calendar Round. In Aztec culture, the end of each calendar round brought great anxiety, and the start of each new 52-year cycle was celebrated with a joyous New Fire ceremony.

“Every 52 years, Aztec people would put out all their fires on the last day of the calendar round, and they would throw out all their household possessions,” Smith says. “Then they would wait, and if the Pleiades rose, that meant the sun was going to rise. Then the priests did a sacrifice outside of Tenochtitlan, and everybody relit their fires.”

According to Spanish accounts, the Aztecs held their last New Fire ceremony in 1507. Nine more 52-year cycles have been completed since then, and the next Calendar Round is scheduled to end in 2027. Plan accordingly.