The xiuhpōhualli (Nahuatl pronunciation: [ʃiʍpoːˈwalːi], from xihuitl (“year”) + pōhualli (“count”)) is a 365-day calendar used by the Aztecs and other pre-Columbian Nahua peoples in central Mexico. It is composed of eighteen 20-day "months," which through Spanish usage came to be known as veintenas (“scores, groups of twenty”), with an inauspicious, separate 5-day period at the end of the year called the nēmontēmi. The name given to the 20-day periods in pre-Columbian times is unknown, and though the Nahuatl word for moon or month, mētztli, is sometimes used today to describe them, the sixteenth-century missionary and ethnographer, Diego Durán explained that:
The xiuhpōhualli calendar (in history known as the "vague year" which means no leap day) had its antecedents in form and function in earlier Mesoamerican calendars, and the 365-day count has a long history of use throughout the region. The Maya civilization version of the xiuhpōhualli is known as the haab', and 20-days period was the uinal. The Maya equivalent of nemontemi is wayeb'. In common with other Mesoamerican cultures the Aztecs also used a separate 260-day calendar (Classical Nahuatl: tonalpōhualli). The Maya equivalent of the tonalpōhualli is the tzolk'in. Together, these calendars would coincide once every 52 years, the so-called "calendar round," which was initiated by a New Fire ceremony.