The Aztec calendar had 18 periods of 20 days each. These periods formed the 360 named days of the 365-day xiuhpōhualli, or solar calendar.
The remaining five days were called nemontēmi. They were added at the end of the year and were considered inauspicious or “nameless” days. Although these periods are often called months in English, they were not lunar months; their original Nahuatl name is uncertain, and modern writers commonly call them veintenas, Spanish for groups of twenty.
The Aztecs also used the tōnalpōhualli, a separate 260-day ritual calendar based on combinations of 20 day signs and 13 numbers. The two calendars aligned every 52 years, an important cycle marked by the New Fire ceremony. Each veintena also had its own festivals and religious associations, linking timekeeping to agriculture, sacrifice, and worship.