Excerpt:
Not just one calendar
There is not just one Aztec calendar, there are two more or less independent systems. One calendar, called the xiuhpohualli, has 365 days. It describes the days and rituals related to the seasons, and therefor might be called the agricultural year or the solar year. The other calendar has 260 days. In Nahuatl, the language of the Aztecs, it is called the tonalpohualli or, in English, the day-count. Most information on this Internet-site refers to the tonalpohualli, which is the sacred calendar.