late 15c., from Old French festal, from Late Latin festalis, from Latin festum "feast" (see feast (n.)).
*dhēs-, Proto-Indo-European root forming words for religious concepts. Possibly an extension of PIE root *dhe- "to set, put."
It forms all or part of: apotheosis; atheism; atheous; Dorothy; enthusiasm; fair (n.) "a stated market in a town or city;" fanatic; ferial; feast; fedora; -fest; festal; festival; festive; festoon; Festus; fete; fiesta; henotheism; monotheism; pantheism; pantheon; polytheism; profane; profanity; Thea; -theism; theist; theo-; theocracy; theodicy; Theodore; Theodosia; theogony; theology; theophany; Theophilus; theosophy; theurgy; tiffany; Timothy.
It is the hypothetical source of/evidence for its existence is provided by: Greek theos "god;" Latin feriae "holidays," festus "festive," fanum "temple."
1580s, "a festal day, appointed day of festive celebration," short for festival day (late 14c.), from Old French festival (adj.) "suitable for a feast; solemn, magnificent, joyful, happy," and directly from Medieval Latin festivalis "of a church holiday," from festum "festival, holiday," neuter of festus "of a feast" (see feast (n.)). The English word returned to French 19c. in certain specialized senses.
c. 1200, "secular celebration with feasting and entertainment" (often held on a church holiday); c. 1300, "religious anniversary characterized by rejoicing" (rather than fasting), from Old French feste "religious festival, holy day; holiday; market, fair; noise, racket; jest, fun" (12c., Modern French fête), from Vulgar Latin *festa (fem. singular; also source of Italian festa, Spanish fiesta), from Latin festa "holidays, feasts, festal banquets," noun use of neuter plural of festus "festive, joyful, merry," related to feriae "holiday" and fanum "temple," from Proto-Italic *fasno- "temple," from PIE *dhis-no- "divine, holy; consecrated place," suffixed form of PIE root *dhes-, forming words for religious concepts.
The spelling -ea- was used in Middle English to represent the sound we mis-call "long e." Meaning "abundant meal" (whether public or private) is by late 14c. Meaning "any enjoyable occasion or event" is from late 14c.