in (one's) 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.
See also: Sunday
Farlex Dictionary of Idioms.

*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.
See also: Sunday
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
See also:
  • in Sunday best
  • Sunday best
  • your Sunday best
  • get into (one's) Sunday best
  • (one's) Sunday best
  • be in (one's) Sunday best
  • best do
  • (had) best (do something)
  • as best (one) can
  • as best you can