"ninth month of the Roman year," late Old English Septembre, from Old French septembre, setembre, and directly 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 original 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. The native names are Old English hærfestmonað, haligmonað. By late 14c. as figurative of as later stage of life. Related: Septembral; Hawthorne has Septemberish. Septembrian as "belonging to September" is by 1800; earlier it was a noun, "one who believes that our Lord was born in September" (1640s).