When I was a young kid "Sunday clothes" was still in vogue
to describe the best suit or dress ( for guys and gals respectively...just to make it clear, I could see the vultures diving in on that ambiguity ) you wore for church or special occasions. I think the term goes back to "The Gilded Age" (late 1800's) when everyone wanted to be the Vanderbilts or Astors but were too poor to afford more than one set of nice clothes.