Why is the ninth month called September?
1580s, "quality of being synchronous," from Modern Latin synchronismus, from Greek synchronismos, from synchronos (see synchronous). Meaning "recurring at the same successive instants of time" is from 1854.
synchronism syn·chro·nism (sĭng'krə-nĭz'əm, sĭn'-)
Coincidence in time; simultaneousness. Also called synchronia.