The tōnalpōhualli, meaning "count of days" in Nahuatl, is a Mexica version of the 260-day calendar in use in pre-Columbian Mesoamerica. This calendar is solar and consists of 20 13-day periods. Each trecena is ruled by a different deity. Graphic representations for the twenty day names have existed among certain ethnic, linguistic, or archaeologically identified peoples.

The Aztecs were a Mesoamerican culture that flourished in central Mexico in the post-classic period from 1300 to 1521. The Aztec people included different ethnic groups of central Mexico, particularly those groups who spoke the Nahuatl language and who dominated large parts of Mesoamerica from the 14th to the 16th centuries. Aztec culture was organized into city-states (altepetl), some of which joined to form alliances, political confederations, or empires. The Aztec Empire was a confederation of three city-states established in 1427: Tenochtitlan, city-state of the Mexica or Tenochca, Texcoco, and Tlacopan, previously part of the Tepanec empire, whose dominant power was Azcapotzalco. Although the term Aztecs is often narrowly restricted to the Mexica of Tenochtitlan, it is also broadly used to refer to Nahua polities or peoples of central Mexico in the prehispanic era, as well as the Spanish colonial era (1521–1821). The definitions of Aztec and Aztecs have long been the topic of scholarly discussion ever since German scientist Alexander von Humboldt established its common usage in the early 19th century.