People in Late Postclassic Central Mexico used a ritual calendar of 260 days. This was called the tonalpohualli_ (“day count”) in Nahuatl. The same basic calendar had been used for centuries throughout Mesoamerica, and is still used in some areas today.⁴ Each day in this cycle had its own distinct name, created by a combination of the numbers 1 to 13 (represented by a series of dots) and one of twenty day signs (represented by small pictures of objects, animals, and natural forces). All twenty of these day signs are arranged in a circle around the center of the Calendar Stone (Figure 4). The cycle begins with a right-facing alligator head (located just below the large pointed sunbeam at the “top” of the monument). This is the sign for _cipactli, “Alligator.” Moving counterclockwise, the next sign in the series is the beak-masked face of the wind god Ehecatl (“Wind”). The third sign is a boxy carving of a building, representing calli (“House”). Next comes Lizard (cuetzpalin), Snake (coatl), Death (miquiztli), Deer (mazatl)…and the whole 20-sign list comes to an end with Flower (xochitl), at which point the series begins again with Alligator.

These twenty signs were combined with thirteen numeric values to create a series of 260 different combinations. The tonalpohualli began on the day 1 Alligator. It was followed by 2 Wind, 3 House, 4 Lizard, 5 Snake, 6 Death—and so on up to 13 Reed. Then, the numeric values began again with the day 1 Jaguar, 2 Eagle, 3 Vulture, 4 Motion, 5 Flint, 6 Rain, and 7 Flower. At this point, the day sign cycle began again with day 8 Alligator. After 260 days had passed, the cycles of 20 and 13 came into alignment again with the day 1 Alligator, and the whole series began again. Below is a chart showing all 260 days of this calendar. It is read from top to bottom and from left to right, beginning in the upper left-hand corner and ending in the lower right-hand corner.

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Several examples of these number-plus-day sign date combinations can be seen on the surface of the Calendar Stone. The day 1 Flint (written using a single round dot for the number 1 and a pointed knife blade for the sign Flint) is located right below the Flower sign in the ring of day signs we just discussed (Figure 5). Moving clockwise, the day 7 Monkey (written using seven dots and a monkey’s head) is located directly above the sign for Monkey in the ring of day signs. Just to the left of the 7 Monkey sign is the sign for the day 1 Rain (written using a single round dot and an image of the head of the Rain god).

This cycle of 260 days was subdivided into twenty thirteen-day periods called trecenas in Spanish (the original Nahuatl name for these periods is unknown). Each trecena was named for the day on which it began, and these days always had the numeric value of 1. For example, the first trecena was named 1 Alligator and the second was named 1 Jaguar. There were twenty trecenas in each 260 day cycle (much in the way that there are 12 months subdividing the 365 day calendar used in many parts of the world today). Each of these thirteen-day cycles was ruled over by a different deity. For example, the second trecena (1 Jaguar) was ruled over by the god Quetzalcoatl, (“Feathered Serpent”).

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⁴ For more information on the Mesoamerican calendar, see Lipp 1991, 61-69; Tedlock 1992; Boone 2007, 14-18; as well as the Keeping Time �udzavui tutorial.