Why is the ninth month called September?
c.1200, from Anglo-French, Old French sumundre "summon," from Vulgar Latin *summundre "to call, cite," from Latin summonere "hint to," from sub "under" + monere "warn, advise" (see monitor (n.)). Summons "authoritative call to be at a certain place for a certain purpose" is late 13c., from Old French sumunse, noun use of fem. past participle of somondre. Summoner "petty officer who cites persons to appear in court" is from early 14c.; contracted form sumner is from mid-14c.