a regularly scheduled day off for a shop or business (a fixed closing day, often the same weekday each week).
この店は毎週水曜日が定休日です。
This shop is regularly closed on Wednesdays.
定休日を確認してから出かけた方が安心だ。
It's safer to check the regular closing day before you go out.
祝日が定休日に当たると、営業する店もあるので注意してください。
If a public holiday falls on a regular closing day, some shops may open, so be aware of that.