VEINTENA

A 'veintena' is a subdivision used in pre-Columbian Mesoamerican calendars, consisting of a 20-day period. The division is often casually referred to as a "month", although it is not coordinated with the lunar cycle. The term is most frequently used with respect to the Aztec calendar, specifically the 365-day cycle approximating the tropical year; however the same divisions are observed in the equivalent Maya calendar (the Mayan tun), as well as those used by other Mesoamerican civilizations such as the Zapotec and Mixtec.
The 365-day cycle is divided into 18 veintenas of 20 days each, giving 360 days; an additional 5 "nameless days" or ''nemontemi'' are appended to bring the total to 365.