In Roman times December was the tenth month, so indicated by decem, meaning 10 (a word that we also use in talking about the decimal system). In 46 B.C.E., Julius Caesar added two days to the month, giving it the 31 days we presently recognize. Sagittarius astrologically dominates the beginning of December, making it a month filled with the energy of change. Come the end of the month, Capricorn joins in, offering far more even-handed vibrations so that we can end the year on a calmer note.