The aztecs had two calendars that determined their religious ceremonies. The most important was the one called tonalpohualli. It was a combination of a series of twenty signs with another series of numbers from 1 to 13, the signs and the numbers being combined in such a way that both series followed an invariable order. The same combination of sign and number was not repeated until 13 times 20 or 260 days had passed.

The series of signs as follows:

Alligator Monkey
Wind Grass
House Reed
Lizard Jaguar
Serpent Eagle
Death King Buzzard (vulture)
Deer Earthquake
Rabbit Flint (knife)
Water Rain
Dog Flower

The series of the thirteen numbers follows the normal order: 1,2,3,4,5,6,7,8,9,10,11,12,13.

By combining both series, one gets "1 Alligator" as the name of the first day; of the second, "2 Wind"; of the third, "3 House"until one reaches the day "13 Reed." The following day is called "1 Jaguar"; the next, "2 Eagle," and so on. When the day "Flower" is reached, it is necessary to start counting the day "Alligator" over again with its corresponding number.

This ritual calendar, or tonalpohualli, is one of the most original developments of the indigenous cultures of Mesoamcrica. It is very ancient, for we find it already in use in Oaxaca at the time of me first culture that flourished in the valley, several centuries before the Christian era, and that is known to us as Monte Alban I. This calendar was essentially the basis for all the other calendar computations, such as the Mayan, the Zapotec, the Mixtec, the Totonac, the Huaxtec, the Teotihuacan, the Toltec, and the Aztec.

All the peoples of Mesoamcrica were familiar with and used this calendar, and the day known in Mexico, for example, by the name "13 Serpent" was likewise known by a similar or corresponding name in the entire Mesoamerican area from Panuco to Nicaragua and from Sinaloa to Yucatan.

The period of the 260 days, or the tonalpohualli, was recorded in special books called tonalamatl, "paper or book of the days." Hence the priests who interpreted its signs and the succession of events according to the propitious days and the evil days were known as the tonalpouque.

We do not know where this ritual calendar originated. It is so important and so characteristic of Mexico and Central America that the Mesoamcrican zone could be called the zone of the tonalpohualli. Its development is without doubt very old, and it must have been the creation of a people who attained a high degree of culture prior to that of all the peoples with whose cultures we are now familiar.