Kayab is glossed as turtle, and the month glyph carries a turtle head in the Classic inscriptions. The turtle is a substantial figure in Maya cosmology, appearing as the cracked carapace of the earth from which the Maize God is reborn in Classic painted scenes, and as a constellation identified with the belt of Orion.
The seventeenth month is the last but one of the eighteen full months, and the year is by this point close to its seam. What follows Kumku is not another month but the five-day Wayeb, and because five is not twenty, the alignment between the Haab positions and the Tzolk'in day signs shifts by five places at the year turn, which is exactly why the four possible Year Bearers stand five apart in the twenty.
A Maya date does not name a month and a number in the way a European one does. It names a day in the 260-day Tzolk'in, by number and sign, and a position in the 365-day Haab, by day and month, and the two together form the Calendar Round, which repeats only after 18,980 days, a little under fifty-two years. Within the Haab the days of a month are numbered from zero to nineteen, and the zero day is not called first but seated, the seating of the month, a usage the Classic inscriptions write with its own glyph.