The Maya discovered an advanced system of astronomy that enabled them to develop intricate calendars. One calendar was for religious ceremonies; the other was based on the moon phases. The Maya believed that the heavenly bodies were gods, and if they understood what the gods were doing, they could predict what would happen on earth. The Maya were so advanced in their knowledge of astronomy that they built observatories thousands of years ago. Many are still standing today, including one that is well preserved in Chichén Itzá.
The Maya year has a basic unit called Kin, a word that means day, or sun. The regular calendar, Uinal, was based on the moon’s phases and helped the Maya judge when to plant, harvest, and keep track of time. This calendar took 365 days of the year and divided them into 18 months of 20 days each (numbered zero through 19). The "leftover" five days (numbered zero through four) were considered unlucky, and all activities were canceled. Each of the 20 days was named, and each of these 20 names had a glyph to represent it. Examples of day names include Waterlily, Wind, Jaguar, Earth, and Corn. Children were named for the day they were born on and were believed to posses the characteristics of that symbol.
Only priests were allowed to have the knowledge of calendars and math. In this way, they were able to keep power over the other citizens. The two calendars were consulted separately as well as together. The cogs (teeth) around the outside of both calendars fit and turned together. The combination of the two gave the priests new information in regard to the relationship of the gods.
Known as the Calendar Round, both tied to a linear count of days, called the Long Count, whose zero point is an unknown mythical event that occurred Aug. 13, 3114 B.C. That date, incidentally, is well before the beginnings of any advanced cultures in the region. The Maya themselves regarded the zero date as only the latest in an infinite series of such zero dates, repeating every 5,000 years or so (guess when the next one is!).
Note: Some Maya calendars have absolute beginning dates, and some count from the beginning of each ruler's reign.
Learn more about Calendar Calculations (PDF).