Mayan astrology rests on the Tzolkin, a 260-day ritual calendar, and its structure is a piece of clean combinatorics.
Twenty named day signs, Imix, Ik, Akbal, Kan, Chicchan, Cimi, Manik, Lamat, Muluc, Oc, Chuen, Eb, Ben, Ix, Men, Cib, Caban, Etznab, Cauac and Ahau in the Yucatec names, run against thirteen numbers or tones. Both cycles advance together each day, so day 1 Imix is followed by 2 Ik and so on; because 13 and 20 share no common factor, every combination occurs once before the pattern repeats, which takes 260 days.
Each day sign carries a character, a patron, a world direction and a colour, and each tone carries a quality of force or intensity. A person born on a given day takes that sign and tone, and daykeepers read the combination for temperament, aptitude and vocation.
The Tzolkin meshes with the 365-day solar year, the Haab, of eighteen months of twenty days plus a five-day remainder. The two calendars return to the same pairing every 18,980 days, which is fifty-two years, the Calendar Round that structured Mesoamerican ritual life. Beyond that the Maya kept the Long Count, an absolute day tally from a base date in 3114 BCE, which is what allowed them to date monuments precisely and is why Maya chronology can be correlated with the Western calendar at all.
Why 260 days is unresolved. Proposals include the approximate human gestation period, the interval between zenith passages of the sun at certain Mesoamerican latitudes, and the cycle of Venus as a morning star; none is established, and the count is attested across Mesoamerica from at least the first millennium BCE, older than the Maya themselves.