The name recorded here as Wo appears in older literature as Uo, and it is the second of the eighteen full months. Its meaning is given as the black conjunction, pairing it with Sip, the red conjunction, which follows it; the colour-paired month names are one of the most distinctive features of the Haab list and they recur later in the year in the four storm months, Chen, Yax, Sak and Keh, black, green, white and red.
Those pairings are not decoration. Colour and direction form a single system in Maya thought, with red for east, white for north, black for west and yellow for south, and green at the centre, so a set of colour-marked month names carries a directional order inside the year. How far that order was operative in the calendar rather than only in the naming is not settled, and the atlas does not assert more than the names themselves support.
Structurally Wo behaves like every other full month. It runs from its seating, written 0 Wo, through 19 Wo, and because it is twenty days long and the Tzolk'in day sign also cycles in twenty, its seating carries the same day sign as the seating of Pop before it and of Sip after it.
Wo also illustrates a difficulty in the sources. The colonial Yucatec spellings recorded in the sixteenth century, Uo among them, were fixed by Spanish ears and a Spanish alphabet, while the Classic glyphs were read centuries later by epigraphers working from the signs themselves. The modern spellings used here follow current orthographic conventions for Mayan languages, so a reader comparing an older book with this one will meet the same month under three spellings and should not take them for three months.