The Calendar Round (CR) consists of 2 basic calendar cycles – a 365-day solar year (SY) and a 260-day divinatory calendar or ritual almanac (RA) – versions of which were used throughout Mesoamerica. The permutation of these two cycles creates the larger CR cycle of 52 years. In the illustration, the current day is 9 Ajaw in the RA and 19 Kumk'u in the SY; the 4th following day is 13 K'an in the RA and 3 Wayeb' in the SY. Note that this is a modern schematic representation of the CR; there is no indication that the ancient Maya conceptualized it this way.

Solar Year (SY)

The haab' is the solar or seasonal year of 365 days, often called the Vague Year because it was not adjusted to the actual length of the solar year (approximately 365.25 days). As a result, each year the calendar day on which seasonal events, such as the solstices, were expected arrived earlier and earlier before the actual event. This slippage – 0.25 days per year, 1 full day every 4 years – was easily perceptible, and the ancient Maya must have taken account of it, but they did not adjust the calendar to correct for it.

The structure of the haab' involves 19 units (“months”), 18 of 20 days each and a 19th of 5 days. Within each month, days were tabulated in terms of elapsed time: the 1st day of each month was the “seating” of that month, often glossed as 0, because a full day had not yet elapsed. The final (20th) day was numbered 19. The 2nd day of Pop, for example, is designated 1 Pop because only one day has fully elapsed. The previous day, the seating of Pop (0 Pop), was the first day of Pop and simultaneously the completion of Wayeb, the previous month. Rarely, the seating day of a given month was designated “completion” of the previous month; conceptually, as in other Maya cycles, the end of one cycle is the beginning of the next. The chart on the right gives the forms of month glyphs used in the codices.

Ritual Almanac (RA)

The Ritual Almanac (RA) is a cycle of 260 days, whose designations are formed by the permutation of the numbers 1-13 and 20 names. It functioned (and still functions) mainly to identify the supernatural forces salient on particular days and to schedule appropriate rituals. It is often called the tzolk’in, which means “count of days” in Yucatec Maya, but it is not certain that the ancient Maya used this term. The chart on the right gives the forms of the tzolk'in day glyphs used in the codices.