This is the world famous Aztec calendar stone. It consists of a 365 day calendar called the Xiuhpohualli and a 260 day ritual cycle called the Tonapohualli. The former is the agricultural cycle based on the sun, and the latter the sacred cycle.
Above is the room in which the calendar stone is held. The National Museum was very ergonomically designed, and houses thousands of priceless treasures from pre-Colombian Mexico. Each distinct culture such as the Aztec, Olmec, Maya etc. has its own section in this vast complex, as well as extensive gardens outside.
The Tonalpohualli or day count consists of a cycle of 260 days, each day signified by a number, from 1 to 13, and one of the 20 day signs seen above. The set of day signs is exactly the same as used by the Mixtec people, and each of the day signs is associated with one of the cardinal directions.
The year is composed of 18 months, each containing 20 days that had names, plus 5 more which were nameless, and regarded as being unlucky.