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Might qualify under Tropes Are Flexible, but this festival is specifically called out as not exclusive or requiring an invitation or formal wear—all members of the public may freely come and go.
Edited by HeraldAlberichLost in a Crowd is the closest.
I think you might have a Missing Supertrope here if you have the gumption for a TLP. I think we have several that woud be subtropes of your general idea.
Edited by DracMonster
A festival or other crowd-gatherer that the protagonists take advantage of. Or more generally, an event that makes the plot easier to pull off.
In Star Wars: Scoundrels, Han Solo's crew attempts to rob a crime lord's mansion during a week-long festival. Since this crime lord likes to play civic-philanthropist (and compete with other rich bigwigs who do the same thing), his grounds are open to the public on each of the four major festival days, providing an opportunity to get past the walls and several other security measures for The Caper. It would be a lot harder to burgle the mansion any other time of year.