The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with 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”).