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The Xiuhpōhualli (Nahuatl pronunciation: [ʃiʍpoːˈwalːI], from xihuitl + pōhualli) 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 veintenas or mētztli (the contemporary Nahuatl word for month) with a separate 5-day period at the end of the year called the nemontemi. 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 veintena. The Aztec word for moon is mētztli, 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 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.
Aztec years were named for the last day of the 18th month according to the 260-day calendar the tonalpōhualli. The first year of the Aztec calendar round was called 2 Acatl and the last 1 Tochtli. The solar calendar is connected to agricultural practices and holds an important place in Aztec religion, with each month being associated with its own particular religious and agricultural festivals. Each 20-day period starts on a Cipactli (Crocodile) day of the tonalpōhualli for which a festival is held. The eighteen veintena are listed below. The dates in the chart are from the early eyewitnesses, Diego Durán and Bernardino de Sahagún. Each wrote what they learned from Nahua informants. Sahagún's date precedes the Durán's observations by several decades and is believed to be more recent to the Aztec surrender to the Spanish. 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.
The 20-day months (veintenas) of the Aztec solar calendar are called (in two sequences):
- Atlcahualo or Xilomanaliztli
- Toxcatl or Tepopochtli
- Tlaxochimaco or Miccailhuitontli
- Xocotlhuetzi or Hueymiccailhuitl
- Teotleco or Pachtontli
- Tepeilhuitl or Hueypachtli
The five days inserted at the end of a year are days of reflection and contemplation still observed today. 
Note: Aztec years were named for the last day of their fourth month according to the 260-day calendar, the tonalpohualli.
Reconstruction of the calendar
For many centuries, scholars have tried to reconstruct the Aztec calendar. A correlation that is accepted in some circles was proposed by professor Rafael Tena (INAH), based on the studies of Sahagún, Durán and Alfonso Caso (UNAM). His correlation argues that the mexica year started on February 13th using the old Julian calendar or February 23rd of the current Gregorian calendar.
- "The Nemontemi and the Month Quahuitlehua in the Aztec Solar Calendar". World Digital Library.
- The Mexica Calendar and the Cronography. Rafael Tena. INAH-CONACULTA. 2008 p 82-83
- The Mexica Calendar and the Cronography. Rafael Tena. INAH-CONACULTA. 2008
- Miller, Mary; Karl Taube (1993). The Gods and Symbols of Ancient Mexico and the Maya. London: Thames and Hudson. ISBN 0-500-05068-6.