Sotz means bat, and its glyph in the Classic inscriptions is a bat head, one of the more immediately legible of the nineteen. The bat is a substantial figure in Maya religious imagery well beyond the calendar, appearing in the Popol Vuh as the killing bat of Zotzilaha, one of the houses of trial in Xibalba, and as an emblem of the Tzotzil, whose name carries the same root.
Within the calendar Sotz is the fourth of the eighteen full months and behaves like the rest of them: twenty days, numbered from the seating at zero through nineteen, meshed with the 260-day Tzolk'in so that any given date is named by both counts at once. The pair repeats only after 18,980 days, the Calendar Round of a little under fifty-two years, and that period is the longest span the Round alone can date without ambiguity.
For anything longer the Classic Maya used the Long Count, a linear tally of days from a fixed starting point, and Classic monuments routinely give a Long Count, a Tzolk'in day and a Haab position together. A date recorded in all three is fixed absolutely, which is why the inscriptions can be correlated with the European calendar at all.
The Calendar Round was not only Maya. Central Mexican peoples ran the same 260-day and 365-day pairing, calling them the tonalpohualli and the xiuhpohualli, and the fifty-two-year completion was marked in the Basin of Mexico by the New Fire ceremony, in which household fires were extinguished and rekindled from a single new flame. The Maya sources record no equivalent ceremony, and the two traditions should not be run together on the strength of the shared arithmetic.