The calendrical systems devised and used by the pre-Columbian cultures of Mesoamerica, primarily a 260-day year, were used in religious observances and social rituals, such as divination.
These calendars have been dated to early as ca. 1100 BCE. By 500 BCE at the latest, the essentials were fully defined and functional. 260-day calendars are still used in the Guatemalan highlands, Veracruz, Oaxaca and Chiapas, Mexico.
The importance of aboriginal calendars in ritual and other aspects of Mesoamerican life was noted by many missionary priests, travelers, and colonial administrators, and later by ethnographers who described and recorded the cultures of contemporary Mesoamerican ethnic groups.
Among the various calendar systems in use, two were particularly central and widespread across Mesoamerica. Common to all recorded Mesoamerican cultures, and the most important, was the 260-day calendar, a ritual calendar with no confirmed correlation to astronomical or agricultural cycles. Apparently the earliest Mesoamerican calendar to be developed was known by a variety of local terms, and its named components and the glyphs used to depict them were culture-specific. However, it is clear that type of calendar functioned in essentially the same way across cultures, and across the chronological periods when it was maintained.
The second of the major calendars was one representing a 365-day period approximating the tropical year, known sometimes as the "vague year". Because it was an approximation, over time the seasons and the true tropical year gradually "wandered" with respect to this calendar, owing to the accumulation of the differences in length. There is little hard evidence to suggest that the ancient Mesoamericans used any intercalary days to bring their calendar back into alignment. However, there is evidence to show Mesoamericans were aware of this gradual shifting, which they accounted for in other ways without amending the calendar itself.
These two 260- and 365-day calendars could also be synchronised to generate the Calendar Round, a period of 18980 days or approximately 52 years.