Approx birth month. First month is the month they're conceived and the one beside it is the approximate month of birth, if they're born full term. Handy guide if you want to try to have a baby in a specific month.
The ancient Celts didn't have tree signs, but they were skilled astronomers and most likely used some form of predictive astrology. We do know they had lunar calendars(e.g.,the Coligny calendar), although scholars disagree as to whether the start of the month was the new moon or the full moon. There is no agreement as to what symbols were used by the druids who created the calendars, but most historians think it is similar to the old Vedic/Hindu cosmological system.