irregular days off; no fixed closing days; no set holidays
Describes a business or workplace where days off are not on a fixed weekly schedule, or a shop that closes irregularly rather than on a set weekday.
This shop has irregular closing days, so you should call ahead before going.
His job has no fixed days off, so he can't always rest on weekends.
不定休 describes a business that has no fixed regular closing day, the opposite of 定休, which means a set weekly day off.
Compound of 不 (fu, 'not') + 定 (tei, 'fixed') + 休 (kyū, 'rest, holiday'), literally 'non-fixed holiday'.