Xiuhpōhualli

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:

In ancient times the year was composed of eighteen months, and thus it was observed by these Indian people. Since their months were made of no more than twenty days, these were all the days contained in a month, because they were not guided by the moon but by the days; therefore, the year had eighteen months. The days of the year were counted twenty by twenty.

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.