xiuhpōhualli

The xiuhpōhualli (, from (“year”) + (“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 (“scores, groups of twenty”), with an inauspicious, separate 5-day period at the end of the year called the . The name given to the 20-day periods in pre-Columbian times is unknown, and though the Nahuatl word for moon or month, , 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.