Decorated or adorned with festoons; covered with garlands or chains of flowers, leaves, or ribbons.
Past participle of 'festoon' (verb), used as an adjective. The word describes something that has been decorated with the hanging ornamental chains characteristic of festoons.
When something is 'festooned' with decorations, it's crossed the line from nicely decorated to gloriously over-the-top—which is exactly why we love describing royal balls and holiday displays this way.
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