The Mayan Calendar
This is the Tzolkin calendar used by the Mayans. It counts in base 20 and combines 20 day names (orange) with 13 numbers (brown), giving 260 unique days. This calendar was used for religious purposes. The other two Mayan calendars were the Haab, with 360 days, for civil purposes and the "Long Count" (pink), with cycles up to 2 days.
C. Thomas, Day Symbols of the Maya Year, Washington, DC: Government Printing Office, 1897 pp. 199-266, Project Gutenberg 18973.
"The Mayan Calendar"
Wolfram Demonstrations Project
Published: March 15 2011