The Maya Calendar is a very sophisticated calendar system used by the ancient Maya. It actually consisted of two calendars, one ritualistic calendar consisting of 260 days (13 times 20 days) called Tzolkin, and a sun calendar consisting of 360 days (18 times 20 days) plus 5 unlucky or unnamed days (where the Tomb Raider Underworld level The Unnamed Days gets its name from) called Haab, approximating the about 365¼ days of the sun year. Combined these two result in a cycle of 52 years after which the same dates repeat themselves. Additionally there was a long count for dates further in the future or past than these 52 years. (For further reading see Wikipedia.)
Tomb Raider Underworld
In Tomb Raider Underworld Lara encounters a Maya Calendar puzzle that opens the entrance to Xibalba. She has to align the two calendar rounds to a specific date - the date when in Maya mythology the door to the underworld opens - to solve the puzzle and enter Xibalba.