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]