The tonalpohualli (day-count or count of days), has been called sacred calendar because its main purpose is that of a divinatory tool. This calendar was registered in the tonalamatl (book of days) from which a tonalpouque (priest) cast horoscopes and predicated favorable and unfavorable days of the cycle.

It divides the days and rituals between the gods. For the Aztec mind this is extremely important. Without it the world would soon come to an end.

According to Aztec cosmology, the universe is in a very delicate equilibrium. Opposing divine forces are competing for power. This equilibrium is in constant danger of being disrupted by shifting powers of the gods, of the elemental forces that influence our life. This struggle cannot be won by any god.

The notion that everything ultimately consists of two opposing forces is essential to the Aztec worldview. The world is always on the brink of going under in a spiritual war, a war of gods competing for supreme power. To prevent this from happening, the gods have been given their own space, their own time, their own social groups, etceteras, to rule over.

The tonalpohualli tells us how time is divided among the gods.

The year comprised of 260 days, each of which was assigned a date by intermeshing one of 20 day-signs, represented graphically with a glyph, and a number from 1 to13, represented by dots so that no two days in the cycle could be confused. The system can be best understood by imagining two wheels that are connected to each other:

1. One wheel has  numbers "1" to "13"
2. The 2nd wheel has 20 symbols

-> In the initial situation, number "1" combines with the first symbol: 1st day of the tonalpohualli
-> Now the wheels start moving and number "2" combines with the 2nd glyph: 2nd day.
-> After 14 days, an Aztec week (trecenas) of 13 days has passed.
-> The wheel with the numbers shows number "1" again.
-> The other wheel now shows the 14th symbol.
-> After 260 days, the two wheels have returned to their initial position.
-> The tonalpohualli starts all over again.

Therefore, the year was made up of 20 13-day weeks, with the first week beginning on 1 -Crocodile and ending on 13-Reed, the second week running from 1-Ocelot to 13-Deaths' Head and so on.