The two calendars were was basically similar. The ritual day cycle was called Tonalpohualli and was formed, as was the Mayan Tzolkin, by the concurrence of a cycle of numerals 1 through 13 with a cycle of 20 day names, many of them similar to the day names of the Maya.

Where the Aztec differed most significantly from the Maya was in their more primitive number system and in their less precise way of recording dates. Normally, they noted only the day on which an event occurred and the name of the current year. This is ambiguous, since the same day, as designated in the way mentioned above, can occur twice in a year. Moreover, years of the same name recur at 52-year intervals, and Spanish colonial annals often disagree as to the length of time between two events. Other discrepancies in the records are only partially explained by the fact that different towns started their year with different months. The most widely accepted correlation of the calendar of Tenochtitlán with the Christian Julian calendar is based on the entrance of Cortés into that city on November 8, 1519, and on the surrender of Cuauhtémoc on August 13, 1521. According to this correlation, the first date was a day 8 Wind, the ninth day of the month Quecholli, in a year 1 Reed, the 13th year of a cycle.

The Mexicans, as all other Meso-Americans, believed in the periodic destruction and re-creation of the world. The "Calendar Stone" in the Museo Nacional de Antropología (National Museum of Anthropology) in Mexico City depicts in its central panel the date 4 Ollin (movement), on which they anticipated that their current world would be destroyed by earthquake, and within it the dates of previous holocausts: 4 Tiger, 4 Wind, 4 Rain, and 4 Water.

The Aztec calendar kept two different aspects of time; tonalpohualli and xiuhpohualli. Each of these systems had a different purpose. The tonalpohualliwas the 'counting of days.' It originated by ancient peoples observing that the sun, crossed a certain zenith point near the Mayan city of Copan, every 260 days. So this first system is arranged in a 260-day cycle. These 260 days were then broken up into 20 periods, with each period containing 13 days, called trecenas. Each period was given the name of something that was then shown by a hieroglyphic sign, and each trecena was given a number 1-13. Each trecena is also thought to have a god or deity presiding over each of the trecena. They kept these counts in tonalamatls, screenfold books made from bark paper. The Aztecs used this as a religious calendar. Priests used the calendar to determine luck days for such activities as sowing crops, building houses, and going to war.

The xiuhpohualli was the 'counting of the years.' This calendar was kept on a 365-day solar count. This was also the agricultural and ceremonial calendar of the Aztec state. It was divided into 18 periods, with each period containing 20 days, called veintenas. This left five days that were not represented. These were called "nemontemi." These were the five transition days between the old and the new year, and were considered days of nothing. This was a time of festivals. People came to the festivals with their best clothes on, and took part in singing and dancing. This is also when the priest would perform sacrifices, most of these sacrifices were human, but others were preformed on animals and fruit.

The solar year was the basis for the civil calendar by which the Mexicas (Aztecs) determined the myriad ceremonies and rituals linked to agricultural cycles. The calendar was made up of 18 months, each lasting 20 days. The months were divided into four five-day weeks. The year was rounded out to 365 days by the addition of the five-day nemontemi (empty days), an ominous period marked by the cessation of normal activities and general abstinence. The correlation of dates in the Gregorian calendar is uncertain, although most authors on the subject affix the beginning of the Aztec year to early February. A variety of sources were consulted in developing the following chart of some of the ritualistic activities associated with each month.

Many of the Aztecs' religious ceremonies, including frequent human sacrifices, were performed at the Great Temple, located in the center of their capital city of Tenochtitlan.