Why is the ninth month called September?
1570s, "bottom or foundation (of something material)," from Latin basis "foundation," from Greek basis "a step, stand, base, that whereon one stands," from bainein "go, step" (see come). Transferred and figurative senses (of immaterial things) are from c.1600.
basis ba·sis (bā'sĭs)
n. pl. ba·ses (-sēz')
The foundation upon which something, such as an anatomical part, rests.