In the earlier days of English, the word festival referred specifically to religious holidays, so it's changed a tiny amount since then. First recorded in 1589, this came pretty much without event from the Old French word festival, which in turn came from Latin festivus, meaning "lively" or "festive" in general. This can be conjugated to festus, which essentially meant "anything having to do with feasts". Further back, we're already reconstructing: in this case from Proto-Italic festos, from Proto-Indo-European dehs, meaning "deity", because feasts were held to honor the gods. As you may have guessed, this is related to the word feast as well. The latest common root is festus, which gave way to Old French feste, also meaning "religious holiday", which became feast as we know it. In English today, the word festival is used a little more often than feast, but both have been decreasing in utilization since the nineteenth century.
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