Why is the ninth month called September?
1650s, "pertaining to a feast," from Latin festivus "festive, joyous, gay," from festum "festival, holiday," noun use of neuter of adjective festus (see feast (n.)).
Meaning "mirthful" is attested by 1774. Unattested from 1651 to 1735; modern use may be a back-formation from festivity. Related: Festively; festiveness.