Why is the ninth month called September?
mid-14c., "steadfastness," from Old French constance "steadfastness, permanence" (14c.), from Latin constantia (source of Italian costanza, Spanish constancia), noun of action from constantem (see constant (adj.)). Obsolete since 17c. except as a given name for a girl, which enjoyed a mild popularity in U.S. c.1945-1955.