The Aztecs, who dominated Mesoamerica for more than two centuries during the late Middle Ages, had a sophisticated system of keeping track of the days. In fact, they had two calendars, which reconciled relatively regularly.

The 365-day calendar, or xiuhpohualli, was known as the agricultural calendar because the Aztecs based it on the progress of the Sun. A solar year was a xihuiti; a day was a tonalli. This calendar had "months" (veintena) that contained 20 days, each of which had its own symbol. The year had 18 of those months, and the remainder was five days (nemontemi), which occurred as a bloc at the end of the calendar year and had no associated symbols.

Each of those 20-day groupings had a name:

Number	Name	Translation
1	Atlcahualo	stopping of the water
2	Tlacaxipeualiztli	flaying of men
3	Tozoztontli	lesser vigil
4	Hueytozoztli	great vigil
5	Toxcatl	drought
6	Etzalqualiztli	eating maize and beans
7	Tecuilhuitontli	lesser feast of the lords
8	Hueytecuilhuitl	great feast of the lords
9	Tlaxochimaco	offering of flowers
10	Xocotlhuetzi	the fruit falls
11	Ochpaniztli	sweeping
12	Teotleco	return of the gods
13	Tepeilhuitl	feast of the mountains
14	Quecholli	a bird
15	Panquetzaliztli	raising of the quetzal-feather banners
16	Atemoztli	falling of water
17	Tititl	not known
18	Izcalli	growth

Coatl

Ocelotl

The other calendar, the tonalpohualli, had 260 days and tracked religious ceremonies. Each day on this calendar was a combination of a number from 1 to 13. After 20 repetitions of that 13-day set (called a trecena), the calendar turned over. Each of those 20 trecenas had its own associated number, name, symbol, and god/goddess.

Number	Name	Symbol	Deity
1	cipactli	crocodile	Tonacatecuhtli
2	ehecatl	wind	Quetzalcoatl
3	calli	house	Tepeyolohtli
4	cuetzpallin	lizard	Huehuecoyotl
5	coatl	snake	Chalchuitlicue
6	miquiztli	death	Tecciztecatl
7	mazatl	deer	Tecciztecatl
8	tochtli	rabbit	Mayahuel
9	atl	water	Xiuhtecuhtli
10	itzcuintli	dog	Mictlantecuhtli
11	ozomatli	monkey	Xochipilli
12	malinalli	dead grass	Patecatl
13	acatl	reed	Tezcatlipoca
14	ocelotl	ocelot	Tlazolteotl
15	quauhtli	eagle	Xipe Topec
16	cozcaquauhtli	vulture	Itzpapalotl
17	ollin	earthquake	Xolotl
18	tecpatl	flint knife	Tezcatlipoca
19	quiahuitl	rain	Tonatiuh
20	xochitl	flower	Xochiquetzal

The two calendars joined up every 52 years. It was common practice to offer sacrifices to the gods at this time.

The Aztec sun stone, a representation of these calendars, dates to the early 16th Century. The massive stone (measuring more than 11 feet in diameter, more than 3 feet thick, and weighing more than 54,000 pounds) disappeared after the Spanish conquest of the Aztec Empire; workers doing repair work on Mexico City Cathedral found the stone again in 1790 and were directed to affix it to a wall of the cathedral. There it stayed until 1855, when it gained a new home, the Archaeological Museum. The stone reached its current home, the National Museum of Anthropology and History, in 1964.