Long before the Spanish arrival in Central Mexico, the Mexica and neighboring civilizations measured the sacred rhythm of time through a sophisticated 260-day calendar known as the Tōnalpōhualli, or "count of days" in Nahuatl. Operating alongside the 365-day solar agricultural cycle called the Xiuhpōhualli, this sacred almanac functioned through a seamless mathematical interplay between twenty distinct day signs and a cycle of thirteen numbers. Because thirteen and twenty share no common factors, every single day possessed an entirely unique combination of sign and number, repeating only after a full 260-day cycle had elapsed.

Rather than serving as arbitrary symbols, each of the twenty glyphs was deeply woven into Mesoamerican cosmology and the physical world. The cycle begins with Cipactli (Crocodile), the primordial earth monster torn apart by creator deities to form the physical world. It progresses through natural forces like Ehēcatl (Wind), whose duck-billed mask symbolizes the breeze that sets cosmic motion going, and sacred animals like Itzcuīntli (Dog), the spiritual guide believed to lead human souls across subterranean rivers into Mictlān. Crucially, the sign Ōllin (Movement) represents the current world age known as the Fifth Sun, depicted at the center of the famous Sun Stone and prophesied to culminate in massive earthquakes.

Central to this system was the concept of tonalli, an animating force and cosmic energy that infused each specific day with its own destiny and character. Specialist priests called tonalpouhqueh consulted sacred books of days to read these divine influences, guiding individual names, agricultural endeavors, and societal ceremonies. Far from being an exclusively Aztec invention, this 260-day count represents a shared pan-Mesoamerican heritage used by the Zapotec, Mixtec, and Maya for over two millennia. Every 52 years, when the 260-day ritual count realigned with the 365-day solar year, Mesoamericans celebrated the Calendar Round through the New Fire ceremony, binding time anew and ensuring the continuation of the universe.