1650s, "pertaining to a feast," from Latin festivus "festive, joyous, gay," from festum "festival, holiday," noun use of neuter of adjective festus "joyful, merry" (see feast (n.)). The word is unattested in English from 1651 to 1735 (it reappears in a poem by William Somervile, with the sense "fond of feasting, jovial"), and the modern use may be a back-formation from festivity. Meaning "mirthful, joyous" in English is attested by 1774. Related: Festively; festiveness.
When the Day crown'd with rural, chaste Delight
Resigns obsequious to the festive Night;
The festive Night awakes th' harmonious Lay,
And in sweet Verse recounts the Triumphs of the Day.
[Somervile, "The Chace"]
Earlier adjectives in English based on the Latin word were festival "pertaining to a church feast" (late 14c.); festful "joyous" (early 15c.), festial "pertaining to a church feast" (early 15c.), festli "fond of festivity" (late 14c.).