Yours, Etc.: Origins and Uses of 8 Sign-Offs
late Old English, from Latin September (also source of Old French Septembre, Spanish Setiembre, Italian Settembre, German September), from septem "seven" (see seven). So called because it was the seventh month of the old Roman calendar, which began the year in March; Julian calendar reform (46 B.C.E.) shifted the new year back two months. For -ber suffix, see December. Replaced Old English hærfestmonað, haligmonað. Related: Septembral.
ninth month of the Gregorian calendar. Its name is derived from septem, Latin for "seven," an indication of its position in the early Roman calendar.