September
late Old English, from Latin, from septem ‘seven’ (being originally the seventh month of the Roman year).
wiktionary
From Middle English, from late Old English, from Old French septembre, Latin september(“seventh month”), from septem(“seven”), from Proto-Indo-European *septḿ̥(“seven”); + Latin -ber, from -bris, an adjectival suffix; September was the seventh month in the Roman calendar.
etymonline
September
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.
