The Aztec calendar was the calendar of the Aztec people of Pre-Columbian Mexico. It is one of the Mesoamerican calendars, sharing the basic structure of calendars from throughout ancient Mesoamerica. This calendar is recorded as a carving on the Aztec Calendar Stone currently found in the National Museum of Anthropology and History located within Chapultepec Park, Mexico City.

The calendar consisted of a 365 day calendar cycle and a 260 day ritual cycle. These two cycles together formed a 52 year "century", sometimes called the "Calendar Round".The calendric year began with the first appearance of the Pleiades asterism in the east immediately before the dawn light.

Every month had its name, and the days of the month were numbered from one to twenty. The days of the last month, Nemontemi, were numbered from one to five.The box at the top of the stone contains the stone's year of creation, in this case 1479 CE.

The solar calendar of 365 days was inseparable from the Sacred Round, or Sacred Almanac. The priests used this ritual calendar of 260 days called Tonalpohualli primarily for divinatory purposes. The method of naming the individual days consisted in the combination of twenty pictorial signs with the numbers one to thirteen. Each of the day signs also bears an association with one of the four cardinal directions.The 20 day signs are depicted in the calendar image to the right. They are arrayed in a circle surrounding the central face.

The original Aztec Calendar is a 12', massive stone slab, carved in the middle of the 15th century. Many renditions of it exist and have existed through the years and throughout Mexico.

Historically, the Aztec name for the huge basaltic monolith is Cuauhxicalli Eagle Bowl, but it is universally known as the Aztec Calendar or Sun Stone. It was during the reign of the 6th Aztec monarch in 1479 that this stone was carved and dedicated to the principal Aztec deity: the sun. The stone has both mythological and astronomical significance. It weighs almost 25 tons, has a diameter of just under 12 feet, and a thickness of 3 feet.

On December 17th, 1790 the stone was discovered, buried in the "Zocalo" (the main square) of Mexico City. The viceroy of New Spain at the time was don Joaquin de Monserrat, Marquis of Cruillas. Afterwards it was embedded in the wall of the Western tower of the metropolitan Cathedral, where it remained until 1885. At that time it was transferred to the national Museum of Archaeology and History by order of the then President of the Republic, General Porfirio Diaz.

Xiuhmolpilli commemorative sculpture marking the completion of the fifty-two-year cycle. Every 52 years the tonalpohualli and the xiuhpohualli calendars would align. This marked what was known as a Mesoamerican "century." Every one of these centuries was marked by xiuhmolpilli - Binding Up of the Years or the New Fire Ceremony. This was a festival that lasted 12 days and included fasting as a symbol of penitence. At the beginning of this festival all the lights in the city were extinguished - people let their hearth fires go out. Then on midnight of the 12th day of the festival, a prisoner was taken to the priest. The priest would watch in the night sky for the star of fire to reach the zenith. Once it did, the priest would remove the heart of this man, and replace it with a piece of wood, that was laid on a piece of turquoise. This is where the priest would start the new fire that would once again light the city.

The tonalpohualli (count of days) was the sacred almanac of the Mexicas. This ritual calendar was registered in the tonalamatl (book of days), a green-fold bark paper or deerskin codex from which a priest (called tonalpouque) cast horoscopes and predicated favorable and unfavorable days of the cycle. The almanac year comprised of 260 days, each of which was assigned a date by intermeshing one of 20 day-signs, represented graphically with a glyph, and a number from 1 to13, represented by dots so that no two days in the cycle could be confused. The almanac year was thus made up of 20 13-day weeks, with the first week beginning on 1-Crocodile and ending on 13-Reed, the second week running from 1-Ocelot to 13-Deaths' Head and so on. A god or goddess was believed to preside over each day-sign, as shown in the following chart.

Tonatiuh's Face is the face of the sun, Lord of Heaven, around which takes place all daily and periodic phenomena. The crown, nose-pendant, ear-rings and necklace are magnificent, as must be the ornaments characteristic of this deity. The hair is blond, due to the golden appearance of the sun. The wrinkles on the face show age and maturity. And the tongue, stuck out is the form of an obsidian knife, indicates that the diety demands to be fed with blood and human hearts.

First Ring - from Center. Four Olin representing the Earthquake Epoch or Sun. The four epochs represented inside the square portions of this symbol correspond to the four previous epochs also called suns.

Second Ring - The second ring from the center is composed of 20 named days contained in one month, also used for naming years. Each year starts on one of four of these 20 days.

Snake - Coatl
Lizard - Cuetzpallin
House - Calli
Wind - Ehecatl
Crocodile - Cipactli
Flower - Xochitl
Rain - Quiahuitl
Flint - Tecpatl
Movement - Ollin
Vulture - Cozcacuauhtli
Eagle - Cuauhtle
Jaguar - Ocelotl
Cane - Acatl
Herb - Malinalli
Monkey - Ozomatli
Hairless Dog - Itzquintli
Water - Atl
Rabbit - Tochtli
Deer - Mazatl
Skull - Miquiztli

Third Ring - Sun Rays - Chalchihuite Ornaments - Splashed Blood Symbols

Outer Ring - Dedication Plate - Herbs with Buds - White Scrolls - Flame Sign - Xiucoatl's Tail