black day . black day {n.} A day of great unhappiness; a disaster. It was a black day when our business venture collapsed. Categories: add yours business disaster {n.} Source: A Dictionary of American Idioms by A. Makkai, M.T. Boatner, J.E. Gates © 2024 To section: B Subscribe to: American IdiomsIdioms feedEnglish languageHome page and general feed Add my comment