Muwan means owl, or more precisely a particular bird identified in the inscriptions with a supernatural avian figure, and the month glyph carries a bird head. The Muwan bird appears in Classic iconography well outside the calendar, associated with the sky and with rain.
The fifteenth month is one of the four that stand between Kankin and the closing Wayeb, and like every full month it runs twenty days from a seating written as zero. The seating glyph, read as chum, is used across the eighteen months without distinction, so the notation for the first day of Muwan is formed exactly as for the first day of Pop.
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.
The uniformity of the seating notation is worth dwelling on. Every one of the eighteen full months is seated in the same way, and the five days of the Wayeb are numbered without a seating of their own in the usual analysis, which is one of the formal marks that the closing period was not treated as a month. Its name and its five days both set it apart from the eighteen.