Yax is the green storm, the second of the four colour-marked months that run from Chen through Keh. The word yax is one of the most productive in Classic Mayan, meaning green, blue, new and first at once, and it appears throughout the inscriptions in names, titles and place names as well as in this month and in Yaxkin.
Green sits at the centre of the Maya directional colour scheme rather than at any of the four quarters, which makes Yax the odd member of the storm month group and may be why it stands second rather than first. That is an observation about the names and not a claim about how the months were used, and no colonial or Classic source explains the ordering.
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 Haab month names present a general problem worth stating once. They are recorded in colonial Yucatec, they are attested by glyph in Classic Mayan, and the two languages are separated by centuries; a name whose meaning is transparent in one may be opaque in the other. Yax is among the clearer cases because the root survives in both, but several of the nineteen are glossed with more confidence in the literature than the evidence carries.