The Aztec calendar stone, Mexica sun stone, Stone of the Sun (Spanish: Piedra del Sol), or Stone of the Five Eras, is a large monolithic sculpture that was excavated in the Zócalo, Mexico City's main square, on December 17, 1790. It was discovered whilst Mexico City Cathedral was being repaired. The stone is around 12 feet across and weighs about 24 tons.

The exact purpose and meaning of the stone is unclear. However archaeologists and historians have proposed a number of theories and it seems likely that there are many aspects to the stone.

One aspect of the stone is its religious significance. One theory is that the face at the centre of the stone represents Tonatiuh, the Mexica god of the sun. It is for this reason that the stone became known as the "sun stone". Richard Townsend proposed a different theory, claiming that the figure at the centre of the stone actually represents Tlaltecuhtli, the Mexica sea or earth monster who features in Mexica creation myths. Another feature of the stone relates to time, hence the name "calendar stone". Some of the circles of glyphs are the glyphs for the days of the month. Further, some of the symbols may represent the five ages that the Mexica believed the earth had passed through. Yet another characteristic of the stone may be its geographic significance. The four points may relate to the four corners of the earth or the cardinal points. The inner circles may express space as well as time. Moreover, there is the political aspect of the stone. It may have been intended to show Tenochtitlan as the centre of the world and therefore as the centre of authority.

The Aztec calendar is the calendar system that was used by the Aztecs as well as other Pre-Columbian peoples of central Mexico. It is one of the Mesoamerican calendars, sharing the basic structure of calendars from throughout ancient Mesoamerica.

The calendar consisted of a 365-day calendar cycle called xiuhpohualli (year count) and a 260-day ritual cycle called tonalpohualli (day count). These two cycles together formed a 52-year "century," sometimes called the "calendar round". The xiuhpohualli is considered to be the agricultural calendar, since it is based on the sun, and the tonalpohualli is considered to be the sacred calendar.

The calendric year may have begun at some point in the distant past with the first appearance of the Pleiades (Tianquiztli) asterism in the east immediately before the dawn light. (See heliacal rising.) But due to the precession of the Earth's axis, it fell out of favor to a more constant reference point such as a solstice or equinox. Early Spanish chroniclers recorded it being celebrated in proximity with the Spring equinox.