Sunday Best Meaning

(idiomatic) A person's finest clothing, especially the clothes one reserves to wear to church on Sunday.

Example: 1886, Louisa May Alcott, Jo's Boys, ch. 18:
  It took all the power and skill of that energetic woman to get her son into his Sunday best.
2009, Brian Knowlton, "Economy and Iraq are set to lead Obama agenda," International Herald Tribune, 18 Jan. (retrieved 20 Jan. 2009):
  More than 250 people, most of them African-American churchgoers dressed in their Sunday best, erupted in screams when the presidential motorcade turned onto the street.