Why is the ninth month called September?
early 13c., "to substitute one for another; to make (something) other than what it was" (transitive); from late 13c. as "to become different" (intransitive), from Old French changier "to change, alter; exchange, switch," from Late Latin cambiare "to barter, exchange," from Latin cambire "to exchange, barter," of Celtic origin, from PIE root *kemb- "to bend, crook" (with a sense evolution perhaps from "to turn" to "to change," to "to barter"); cf. Old Irish camm "crooked, curved;" Middle Irish cimb "tribute," cimbid "prisoner;" see cant (n.2). Meaning "to take off clothes and put on other ones" is from late 15c. Related: Changed; changing. To change (one's) mind is from 1610s.
c.1200, "act or fact of changing," from Anglo-French chaunge, Old French change "exchange, recompense, reciprocation," from changier (see change (v.)).
Meaning "a different situation" is from 1680s. Meaning "something substituted for something else" is from 1590s. The financial sense of "balance returned when something is paid for" is first recorded 1620s; hence to make change (1865). Bell-ringing sense is from 1610s. Related: changes. Figurative phrase change of heart is from 1828.