The Aztec calendar is actually a system of 2 calendars: the "xiuhpohualli," or solar calendar, of 365 days, and the "tonalpohualli," or ritual calendar, of 260 days used by the priests to forecast the future. These 2 calendars coincided once every 52 years, which was marked by rituals that included the sacrifice of captured warriors to relight the sun and begin the next cycle.

Reading the Ritual Calendar

Divide the ritual period of 260 days into 20 13-day units called "trecena." Designate a number from 1 to 13 for each day in the trecena.

Assign a symbol (day-sign, glyph) dedicated to a god to each of the 20 cycles.

Read the date by reading the number and the glyph. For example, "1- Cuetzpallin (Lizard)" is the first day of a trecena aligned to the lizard glyph.

Record the date in a folded paper book called a "tonalamatl." Use dots for the number and place them next to the glyph.

Advance both the number count and the day-sign for each successive day. Thus, "1-Cuetzpallin (Lizard)" is not followed by "2-Lizard," but "2-Coatl (Snake)."

Use this calendar to determine lucky days for planting or going to war.

Reading the Solar Calendar

Divide a 365-day year into 18 periods of 20 days each, called "veintenas," with the remaining five days, the "nemontemi," left over as a period of feasting. Subdivide these months into four five-day weeks.

Assign a number to each day within a veintena and a name to each veintena from the following list: Atlacacauallo, Tlacaxipehualiztli, Tozoztontli, Hueytozoztli, Toxcatl, Etzalcualiztli, Tecuilhuitontli, Hueytecuihutli, Tlaxochimaco, Xocotlhuetzin, Ochpaniztli, Teoleco, Tepeihuitl, Quecholli, Panquetzaliztli, Atemoztli, Tititl and Izcalli.

Read the date as a number followed by the month name, e.g., "7-Toxcatl" is the seventh day of the fifth month. Unlike the ritual calendar, "7-Toxcatl" is followed by "8-Toxcatl."

Name the year for the day-sign of the ritual calendar that falls on the last day of the solar calendar year (the fifth day of the nemontemi period). Because the solar calendar year is divisible by 5, only 4 of the 20 day-signs can give their name to years: Calli (House), Tochtli (Rabbit), Acatl (Cane) and Tecaptl (Flint Knife).

Increment the year count by one for each successive year. As with the ritual calendar system, the day-sign changes with each year, so that "1-Calli (House)" is followed by "2-Tochtli (Rabbit)." Record the civil date in the same manner as the religious date, enclosing the year date in a square.

Restart the numeric count after 13, so that the year "13-House" is followed by "1-Rabbit." A complete cycle, or calendar round, is made every 52 years.

Combine the dates from the ritual and civic calendars to specify a particular date. The date Cortez reached Tenochtitlan (Mexico City), Nov. 8, 1519, in the Aztec calendar is 8-Ehecatl (Wind), 9-Quecholli in the year 1-Acatl (Reed), 13th year of a cycle. Use this calendar for seasonal planting, such as for determining when to plant crops.