Why is the ninth month called September?
late 14c., present participle adjective from seem. Seemingly in sense of "to all appearances" recorded from 1590s.
c.1200, "to appear to be;" c.1300, "to be fitting, be appropriate, be suitable," though the more recent sense in English is the etymological one; from Old Norse soema "to honor; to put up with; to conform to (the world, etc.)," verb derived from adjective soemr "fitting," from Proto-Germanic *somi- (cf. Old English som "agreement, reconciliation," seman "to conciliate," source of Middle English semen "to settle a dispute," literally "to make one;" Old Danish some "to be proper or seemly"), from PIE *som-i-, from root *sem- "one, as one" (see same). Related: Seemed; seeming.