Aztec calendar stone, City of Mexico

Aztec calendar, dating system based on the Mayan calendar and used in the Valley of Mexico before the destruction of the Aztec empire. Like the Mayan calendar, the Aztec calendar consisted of a ritual cycle of 260 days and a 365-day civil cycle. The ritual cycle, or tonalpohualli, contained two smaller cycles, an ordered sequence of 20 named days and a sequence of days numbered from 1 to 13. The 13-day cycle was particularly important for religious observance, and each of the 20 numbered cycles within the ritual year was associated with a different deity. Similarly, each named day was associated with a unique deity, and scholars believe that the combinations of ruling deities were used for divination. The civil year was divided into 18 months of 20 days each, plus an additional 5 days called nemontemi and considered to be very unlucky. Again as in the Mayan calendar, the Aztec ritual and civil cycles returned to the same positions relative to each other every 52 years, an event celebrated as the Binding Up of the Years, or the New Fire Ceremony. In preparation, all sacred and domestic fires were allowed to burn out. At the climax of the ceremony, priests ignited a new sacred fire on the breast of a sacrificial victim, from which the people rekindled their hearth fires and began feasting.