Information related to the Maya calendar was transmitted to us by the Maya themselves in the form of numerous inscriptions engraved in stone on temples, steles and other buildings from the pre-Columbian era. Like other civilizations in Mesoamerica, the Maya used two types of calendars, namely the tzolkin, a ritual calendar, and the haab, a solar calendar. They commonly used a third type of dating called the “long count”, the starting point corresponding to the creation date of the current world. This long count is an ancient dating system characteristic of the Maya civilization of the Classic period. The combination of the 365-day haab and of the 260-day tzolkin leads to a cycle of 52 vague years or 73 ceremonial cycles. A day is defined in this cycle, by its position simultaneously in the haab and the tzolkin, this period of time being referred to as the “Calendar round.” The text of this chapter discusses also the relationship between the Maya calendar and our Gregorian calendar which is determined from a number called “correlation” corresponding to the number of days to add to the Maya long count to obtain the Julian day used in astronomy. In the Maya cosmogony, all living beings, not only human beings but also animals and plants, have their nahual, which reflects the dual aspect of the forces composing the Nature, a briefly discussed subject.