Wiktionary says > From Middle English festival (adjective), from Old French festival (“festive”), from Late Latin fēstīvālis, from Latin fēstīvus (“festive”).

Wiktionary says > From Middle English festival (adjective), from Old French festival (“festive”), from Late Latin fēstīvālis, from Latin fēstīvus (“festive”). By surface analysis, festive +‎ -al. Displaced native Old English frēols. The noun is shortened from festival day, from Middle English festival dai, festiuall day (“feast day, festival”). https://en.wiktionary.org/wiki/festival https://en.wiktionary.org/wiki/festival