Why is the ninth month called September?
c.1050, from Latin October (mensis), from octo "eight," from PIE root *octo(u)- "eight" (see eight). Eighth month of the old Roman calendar (pre-46 B.C.E.), which began the year in March. For -ber see December. Replaced Old English winterfylleð. In Russian history, October Revolution (in which the Bolsheviks overthrew the Provisional Government) happened Nov. 7, but because Russia had not at that time adpoted the Gregorian calendar reform, this date was reckoned there (Old Style) as Oct. 25.
10th month of the Gregorian calendar. Its name is derived from octo, Latin for "eight," an indication of its position in the early Roman calendar.