Why is the ninth month called September?
c.1300, aueril, from Old French avril (11c.), from Latin (mensis) Aprilis "(month) of Venus," second month of the ancient Roman calendar, dedicated to the goddess Venus and perhaps based on Apru, an Etruscan borrowing of Greek Aphrodite. In English in Latin form from mid-12c. Replaced Old English Eastermonað, which was similarly named for a fertility goddess. Re-spelled in Middle English on Latin model (apprile first attested late 14c.).
fourth month of the Gregorian calendar. Its name probably derives from the Latin aperire ("to open"), a possible reference to plant buds opening at this time of year in Rome