A Silver Kilo coin was first released in 2007, displaying a beautiful design of the Aztec Calendar. The Aztec Calendar design originates from the calendar system that was used by the Aztecs, consisting of a 365 day calendar cycle, and a 260 day ritual cycle. These two cycles combine together to form a 52 year century. The 365 day calendar cycle is known as “xiuhpohualli”, or the “counting of the years”, based on the solar cycle. This calendar was divided into 18 groups of 20 days each. The 260 day ritual cycle, known as “tonalpohualli”, or the “counting of the days”, was most likely based on astronomy. This calendar cycle was broken into units of 20 days, and ran simultaneously with a group of 13 numbered days. The Aztec calendar stone that displays this calendar system also has the face of the Aztec sun god, Tonatiuh, in the center of the stone, surrounded by four square panels honoring previous ages of the world.