Why is the ninth month called September?
late 13c., hennes, from Old English heonan "away, hence," from West Germanic *hin- (cf. Old Saxon hinan, Old High German hinnan, German hinnen); related to Old English her "here" (see here). With adverbial genitive -s. The modern spelling (mid-15c.) is phonetic, to retain the breathy -s- (cf. twice, pence). Original sense is "away from here;" of time, from late 14c.; meaning "from this (fact or circumstance)" first recorded 1580s.