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having no fixed days off; having no regular closing days; having no regular holidays
当店は不定休となっております。
Our shop has irregular closing days.
このラーメン店は不定休で、月によって営業日が変わる。
This ramen shop has irregular closing days, and its opening days change from month to month.
不定休の店は、事前にウェブで確認しよう。
For shops with irregular closing days, it's best to check online beforehand.