I live in a city that isn’t close to the coast. A sushi chef told me Japanese restaurants are often closed on Mondays/Tuesdays because seafood shipments usually arrive Monday, and then they need time to prep it all.
Upscale Chinese restaurants often feature their menu around seafood so I would imagine some of them do it for the same reason as well.
This may not be the case for restaurants in cities and areas close to harbors.
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