Xiuhpōhualli
The xiuhpōhualli (, from + ) 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," called or (the contemporary Nahuatl word for month) with a separate 5-day period at the end of the year called the . Whatever name that was used for these periods in pre-Columbian times is unknown. Through Spanish usage, the 20-day period of the Aztec calendar has become commonly known as a . The Aztec word for moon is , and this word is today to describe these 20-day periods, although as the sixteenth-century missionary and early ethnographer, Diego Durán explained: 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 calendar (in history known as the "vague year" which means no leap day) had its antecedents in form and function in earlier Mesoamerican