There is not just one Aztec calendar, there are two more or less independent systems, as follows:

1. The Xiuhpohualli, has 365 days. It consists of 18 months of 20 days each plus 5 extra days. It describes the days and rituals related to the seasons, and therefor might be called the agricultural year or the solar year.
2. The Tonalpohualli in Nahuatl, the language of the Aztecs, (day-count) has 260 days and is the sacred calendar. Priests used the calendar to determine luck days for such activities as sowing crops, building houses, and going to war.

Every 52 years, the Aztec held a great celebration called the Binding Up of the Years or the New Fire Ceremony. Before the celebration, people let their hearth fires go out. At the start of the new 52-year cycle, the priests lit a new fire on the chest of a sacrificial victim. People pricked themselves to add their blood to the sacrifice. Then they relit their hearth fires from the new fire and feasted.

Like the Maya before them, the Aztec Indians of Mexico had many religious ceremonies, including frequent human sacrifices, that were performed at the Great Temple (see below ), located in the center of their capital city of Tenochtitlan (today Mexico City).