Mol is glossed as water and as gathering, and it is the eighth of the eighteen full months. Its glyph is well attested in the Classic inscriptions, where Haab positions are written after the Tzolk'in day in the standard Initial Series formula that opens a dated monument.
That formula is worth describing, because it is how almost everything known about Classic chronology reaches us. A monument opens with a large introductory glyph, then the five Long Count places from baktun down to kin, then the Tzolk'in day, then a run of supplementary glyphs recording the lunar age and other cycles, and finally the Haab position such as a day of Mol. The whole assembly fixes one day absolutely.
Because the Long Count, the Tzolk'in and the Haab are independent counts of different lengths, an Initial Series is heavily redundant, and that redundancy is what allowed the system to be deciphered and what allows a damaged inscription to be reconstructed. A single surviving Haab position can settle which of several possible readings of an eroded Long Count is correct.
A last point about the Haab in the Initial Series. The Haab position is written last in the formula, after the Tzolk'in day and the supplementary series, and the two are separated in the text by material about the Moon. That ordering is consistent across the Classic lowlands and is one of the regularities that allowed the structure of the inscriptions to be worked out long before the language itself could be read.