The Calendar

The Aztec people used two different calendars.  The 260-day ritual calendar (called the tonalpohualli) which was used to keep track of rituals, forecast the future, and to determine which days would be lucky and unlucky for the outcomes of various events.  The 260-day calendar is comprised of two repeating, mesh cycles: a cycle of 20 different day names and a cycle of the numbers 1-13.

Cipactli	Alligator	1	8	2	9
Ehecatl	Wind	2	9	3	10
Calli	House	3	10	4	11
Cuetzpallin	Lizard	4	11	5	12
Coatl	Snake	5	12	6	13
Miquiztli	Death	6	13	7	1
Mazatl	Deer	7	1	8	2
Tochtli	Rabbit	8	2	9	3
Atl	Water	9	3	10	4
Itzcuintli	Dog	10	4	11	5
Ozomatli	Monkey	11	5	12	6
Malinalli	Grass	12	6	13	7
Acatl	Reed	13	7	1	8
Ocelotl	Jaguar	1	8	2	9
Cuauhtli	Eagle	2	9	3	10
Cozcacuauhtli	Vulture	3	10	4	11
Ollin	Movement	4	11	5	12
Tecpatl	Flint Knife	5	12	6	13
Quiahuitl	Rain	6	13	7	1
Xochitl	Flower	7	1	8	2

The table above shows a few cycles of the 260-day calendar, the first few days for example, are 1 Cipactli, 2 Ehecatl, 3 Calli, and so on.  After 260 days, 13 cycles of day names have been paired with 20 cycles of numbers, and the final combination (13 Xochitl) is reached, and the calendar begins again.

The Annual or Solar Calendar (called the xiuhpohualli) consisted of 365 days divided into 18 months of 20 days, with 5 unlucky days to finish out the year.  Each 20-day month was divided into 4 weeks of 5 days, and this gave life a structure much like our 7-day week does.  The weeks of the Annual Calendar:

Atl Cahualo
Tlacaxipehualiztli
Tozoztontli
Huei Tozoztli
Toxcatl
Etzalcualiztli
Tecuilhuitontli
Huei Tecuilhuitl
Tlaxochimaco
Xocotl Huetzi
Ochpaniztli
Teotl Ehco
Tepeilhuitl
Quecholli
Panquetzaliztli
Atemoztli
Tititl
+ The the five unlucky days (called Nen Ontemi)

The 260-day Ritual Calendar was combined with the 365-day Annual Calendar resulting in a major cycle of 18,980 days, or 52 years.  This cycle is referred to as a "calendar round," and every day within this cycle had its own unique combination of entries in the two calendars.  An example being when the Spaniards first entered Tenochtitlan on November 8, 1519.  This was the 9th day of Quecholli in the Annual calendar, and day 8 Ehecatl in the ritual calendar.  This same combination (9 Quecholli 8 Ehecatl) reappears every 52 years (or every calendar round).

The Aztec calendar was more complicated than ours is now, but there are noticeable similarities between our and their annual calendar.