Why is the ninth month called September?
late 14c. (early 14c. as a surname), from Old French meur "mature, fully grown, ripe," hence "discreet," from Latin maturus "mature" (see mature (v.)) [OED]. The de- in this word is of uncertain meaning. Or possibly from Anglo-French demuré (Old French demoré), past participle of demorer "stay," and influenced by meur [Barnhart]. Or from Old French de (bon) murs "of good manners," from murs (Modern French moeurs) [Klein].