Latin for 'seventh month' — two months off since 153 BCE, a fossil of the old March-starting calendar.
The ninth month of the year in the Gregorian and Julian calendars, consisting of 30 days.
From Latin 'September,' derived from 'septem' (seven), because it was the seventh month in the original Roman calendar that began in March. Despite being the ninth month since at least 153 BCE, when the civil year was moved to begin in January, the name was never corrected. Several emperors — including Domitian, who renamed it 'Germanicus,' and Commodus, who renamed it after himself — attempted to change it, but all such renamings were reversed
The emperor Domitian renamed September 'Germanicus' after his military campaigns, and Commodus renamed it 'Amazonius' after his gladiatorial persona — but the Romans so resented imperial month-renaming that both names were scrapped the moment each emperor died, and plain old 'September' survived them all.