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Phrase(s): *in one’s Sunday best
Rur. in one’s best clothes; in the clothes one wears to church. (*Typically: be ~; get [into] ~.) • All the children were dressed up in their Sunday best. • I like to be in my Sunday best whenever I go out. • Let’s get into our Sunday best and go out for dinner.
McGrawhill's American Idioms And Phrasal Verbs