Why is the ninth month called September?
late 13c., "to go up, rise, mount (a horse)," from Old French amonter, from a mont "upward," literally "to the mountain," from Latin ad- "to" (see ad-) + montem (nominative mons) "mountain" (see mount (n.)). Meaning "to rise in number or quality (so as to reach)" is from c.1300. Related: Amounted; amounting.
1710, from amount (v.).