Why is the ninth month called September?
early 15c., "belonging to the usual order or course," from Old French ordinarie "ordinary, usual" and directly from Latin ordinarius "customary, regular, usual, orderly," from ordo (genitive ordinis) "order" (see order (n.)). Its various noun usages, dating to late 14c. and common until 19c., now largely extinct except in out of the ordinary (1893). In British education, Ordinary level (abbrev. O level), "lowest of the three levels of General Certificate of Education," is attested from 1947. Related: Ordinarily.