This is not the Mayan Calendar, as History Channel would have you believe. This is the Aztec Calendar, or the Sun Stone. It was a ceremonial artifact, meant to be lying on the ground, and served as the ritualistic calendar for the priests and the polytheistic Aztec religion.
At the center you have the old sun god, Tonatiuh (toh-nah-tee-ooh), with a tongue shaped like a blade. This symbolized a part of the Aztec creation myth, where the sun was made with a godly sacrifice of blood and flesh. As such, the sun needed blood to exist, the blade being the instrument through which human sacrifices were offered to the glory of the gods so that the sun would rise day after day.
More on The Sun Stone in later posts.