The Xiuhpohualli was a calendar cycle constructed from a count of 365 days, used by the Aztecs and other Nahua peoples from the central Mexican region during the Postclassic period of Mesoamerican chronology. This calendar, also known as the "vague year" to scholars of Mesoamerican historical cultures since it approximated the progression of the solar year (xihuitl in Classical Nahuatl), 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. In common with other Mesoamerican cultures this Aztec version of the 365-day calendar was interlocked with a separate 260-day calendar (Nahuatl name: tonalpohualli), to form the "Calendar Round", a cycle of approximately 52 years. A page from the Hindu calendar 1871-72. ... The word Aztec is usually used as a historical term, although some contemporary Nahuatl speakers would consider themselves Aztecs. ... The Nahua are a group of indigenous peoples of Mexico. ... Mesoamerican chronology The chronology of Pre-Columbian Mesoamerica is usually divided into the following eras: Paleo-Indian Period c. ... The cultural areas of Mesoamerica The term Mesoamérica is used to refer to a geographical region that extends roughly from the Tropic of Cancer in central Mexico down through Guatemala, Belize, Honduras, El Salvador, and Nicaragua to northwestern Costa Rica, and which is characterized by the particular cultural homogeneity... Solar year The period of time required for the earth to make one complete revolution around the sun, measured from one vernal equinox to the next. ... Classical Nahuatl (also known as Aztec, and simply Nahuatl) is a term used to describe the variants of the Nahuatl language that were spoken in the Valley of Mexico — and central Mexico as a lingua franca — at the time of the 16th-century Spanish conquest of Mexico. ... The Pre-Columbian people of Mesoamerica kept track of time with calendars which had ritual and religious meaning. ... The Tonalpohualli,the day-count in English, is the 260 day sacred calendar of early Mesoamericans. ... In the Mesoamerican calendars, Calendar Round dates are composed by interlacing the dates of the Tzolkin 260 day period (eg the Tzolkin) with that of the 365 day period (known in the Maya language as the Haab). ... A year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. ...

The Aztec solar year was divided into 18 "months" of twenty days each, called in spanish veintenas, with an addition of an extra five days at the end. The Aztec years were named by name of the last day of the 18^th month according to the 260-day calendar the tonalpohualli, called the "yearbearer". The yearbearer could be one of the signs Acatl "reed", Tecpatl "flint", Calli "house" or Tochtli "Rabbit", combined with a number from one to thirteen (the trecena cycle). When four times thirteen years (every 52 years) had passed in this way a new calendar round was initiated by a New Fire ceremony. The first year of the Aztec calendar round was called 2 Acatl and the last 1 Tochtli. The solar calendar was connected to agricultural practices and held an important place in Aztec religion, with each month being associated with its own particular religious and agricultural festival. The Tonalpohualli,the day-count in English, is the 260 day sacred calendar of early Mesoamericans. ... A trecena is a 13-day period used in pre-Columbian Mesoamerican calendars, which divides the 260-day calendar into 20 trecena of 13 days each. ... The Aztec glyph for a New Fire ceremony, with the year Two Reed (Ome Acatl). ... Agriculture in Mesoamerica dates to the Archaic period of Mesoamerican chronology (8000-2000 BC). ... The Religion of the Aztecs was a typical Mesoamerican religion combining elements of polytheism, shamanism and animism within a framework of Astronomy and calendrics. ...

The 20-day months (veintenas) of the Aztec solar calendar were called (in sequence):

1. Izcalli
2. Atlcahualo or Xilomanaliztli
3. Tlacaxipehualiztli
4. Tozoztontli
5. Hueytozoztli
6. Toxcatl or Tepopochtli
7. Etzalcualiztli
8. Tecuilhuitontli
9. Hueytecuilhuitl
10. Tlaxochimaco or Miccailhuitontli
11. Xocotlhuetzi or Hueymiccailhuitl
12. Ochpaniztli
13. Teotleco or Pachtontli
14. Tepeilhiuitl or Hueypachtli
15. Quecholli
16. Panquetzaliztli
17. Atemoztli
18. Tititl

The five days inserted at the end of a year and which were considered unlucky:

• Nemontemi

The Maya civilization version of the xiuhpohualli is known to Mayanist archaeologists as the Haab', and the Maya equivalent of the tonalpohualli is the Tzolk'in. 74. ... Mayanist is a term which has been in widespread use from the late 19th century onwards, to refer to scholars who have specialised in research and study of the Central American pre-Columbian Maya civilization. ... The Maya Haab calendar is a 365-day solar calendar whose dates indicate the position of the Sun at noon relative to the zenith over the Yucatan peninsula. ... Tzolkin (in the revised orthography which is now preferred, formerly and commonly tzolkin) is the name bestowed by Mayanist scholars upon the version of the 260-day Mesoamerican calendar which was used by the Maya civilization of pre-Columbian Mesoamerica. ...

References

• Miller, Mary; and Karl Taube (1993). The Gods and Symbols of Ancient Mexico and the Maya. London: Thames and Hudson. ISBN 0-500-05068-6.