Why is the ninth month called September?
mid-14c., "minister of a chapel," from Old French chapelein "clergyman" (Modern French chapelain), from Medieval Latin cappellanus "clergyman," originally "custodian of St. Martin's cloak" (see chapel). Replaced Old English capellane (from the same Medieval Latin source) "clergyman who conducts private religious services," originally in great households, later in military regiments, prisons, etc.
originally a priest or minister who had charge of a chapel, now an ordained member of the clergy who is assigned to a special ministry. The title dates to the early centuries of the Christian church