The sacred calendar, used exclusively by priests, consisted of 13 months of 20 days each. It was a ritual tool: it determined the names of newborns, marked auspicious days for battle, and guided the timing of agricultural cycles.

Alongside it ran the civil calendar—used by the broader population and aligned with the solar year. This one had 18 months of 20 days. The remaining five days of the year were considered unlucky or liminal: a time for fasting, retreat, and abstinence.

Every 52 years, the two calendars would align—an event that marked the completion of a full cosmic cycle in pre-Hispanic timekeeping.