in Sunday best
in (one's) Sunday best
Wearing one's very best clothes, as one would wear to a Sunday church service. Instead of some big party, let's all get dressed up in our Sunday best and go for lunch at a fancy restaurant for my birthday! This is going to be a formal event, so please come dressed in your Sunday best.
Farlex Dictionary of Idioms. © 2022 Farlex, Inc, all rights reserved.
*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.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.