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.
Both are shown to emphasize the fact that the beginning of the Native new year became non-uniform as a result of an absence of the unifying force of Tenochtitlan after the Mexica defeat.
A correlation that is accepted in some circles was proposed by professor Rafael Tena (INAH),[4] based on the studies of Sahagún, Durán and Alfonso Caso (UNAM).