Sunday best

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.

one's best clothing, which one would wear to church. (See also Sunday-go-to-meeting clothes.) We are in our Sunday best, ready to go. I got mud on my Sunday best.

One's finest clothes, as in They were all in their Sunday best for the photographer. This expression alludes to reserving one's best clothes for going to church; indeed, an older idiom is Sunday-go-to-meeting clothes ( meeting here meaning "prayer meeting"). [Mid-1800s]

Dressed in one’s finest clothes. A tucker was an ornamental piece of lace worn by women in the seventeenth and eighteenth centuries to cover the neck and shoulders. A bib was either a fancy frill worn at the front of a man’s shirt or an actual formal shirt front. Their pairing with best dates from the mid-eighteenth century. The word bib appeared in print in America in 1795: “The old gentleman put on his best bib and band [i.e., collar]” (The Art of Courting, Newburyport, Massachusetts). A later locution, dating from the mid-nineteenth century, is one’s Sunday best, also known as Sunday-go-to-meeting clothes. It refers to an era when one’s finery was reserved for church (or “prayer meeting”). These Americanisms sound archaic today. See also gussied up.