Chen is the first of the four storm months, whose names carry the colour terms black, green, white and red in order: Chen the black, Yax the green, Sak the white and Keh the red. The four run consecutively as months nine through twelve, a block of eighty days in the middle of the year marked out by a single naming scheme.
The colours are the Maya directional colours, red for east, white for north, black for west, yellow for south, with green at the centre, and the four storm months use four of the five. Why the sequence runs black, green, white, red rather than in the directional order is not explained by any source, and the atlas does not supply a reason where the evidence does not.
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.