The Aztec Calendar Stone was not a calendar, but most likely a ceremonial container or altar linked to the Aztec sun god, Tonatiuh and its festivities. At its center is the image of the god Tonatiuh, within the sign Ollin, which means movement and represents the last of the Aztec cosmological eras, the Fifth Sun.
Tonatiuh’s hands are claws holding a human heart and his tongue is represented by a flint or obsidian knife indicating the need for sacrifice in order for the sun to continue its movement in the sky. At Tonatiuh’s sides are four boxes with the symbols of the preceding eras, or suns, along with the four directional signs. Tonatiuh’s image is surrounded by a cycle containing calendar and cosmological symbols. This ring contains the signs of the 20 days of the Aztec sacred calendar, called Tonalpohualli, which combined with 13 numbers, made up the sacred 260-day year. A second ring contains boxes each with five dots, standing for the five-day week, and signs probably representing sun rays. Finally, the sides of the disk are carved with two fire serpents which transport the sun god in his daily passage through the sky.
Sources for Aztec Calendar information can be found at http://www.hsp-aztec.com/aztec_calendar_fully_explained.htm