Why is the ninth month called September?
late 14c., "fact or action of setting (something), putting, placing, planting," verbal noun from set (v.).
Meaning "place, location, site" is late 14c. Surgical sense, with reference to broken bones, etc., is from early 15c. In reference to heavenly bodies, from c.1400. Also in Middle English "act of creation; thing created" (c.1400). In reference to mounts for jewels, etc. from 1815; meaning "background, history, environment" is attested from 1841.