定休 (teikyū) means “regular closing day”

More examples

定休を変更する場合は、店頭とウェブサイトで早めにお知らせします。tei kyū o hen kō su ru ba ai wa ten tō to we bu sa i to de haya me ni o shi ra se shi ma su

We will promptly announce any change to the regular closing day at the store and on our website.

Identifies a business's predetermined non operating day rather than a temporary or unexpected closure.

休業日kyūgyōhi: A day when a business does not operate, whether the closure is regular or temporary.

店や施設などが、営業を休むとあらかじめ定めた日。mise ya shi setsu na do ga ei gyō o yasu mu to a ra ka ji me sada me ta hi

The word 定休 (teikyū) joins 定 (tei, "fixed") with 休 (kyū, "rest"), expressing a closure that occurs on a settled schedule.

In a shop listing, 定休 (teikyū) can state a recurring closure without naming a weekday, so the actual day is often supplied elsewhere in the notice.