In a manner relating to or characteristic of a festival; with festive spirit or celebration.
From 'festival' plus the adverbial suffix '-ly,' which converts adjectives and nouns into manner adverbs. This creates a word describing how something is done in a festival-like way.
Adding '-ly' to 'festival' feels slightly awkward because we usually say 'festively' instead, so 'festivally' is the word English speakers rarely need because we have a better option already waiting.
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