The 1929 U.S. stock-market day when the Dow suffered its largest percentage loss was called Black Monday.
Black Monday occurred on 28 October 1929, when the Dow Jones Industrial Average fell 38.33 points, or 12.82%. It was followed by Black Tuesday on 29 October, when the Dow lost another 11.73%. Together, the two sessions marked the most dramatic phase of the Wall Street Crash of 1929.
The crash did not begin on either of those dates. The market had already experienced severe declines on Black Thursday, 24 October, and the panic continued through the following week. The collapse ended the speculative boom of the 1920s and helped usher in the Great Depression, although historians distinguish the market crash from the broader economic downturn.
A common mix-up is to identify Black Tuesday as the biggest percentage fall. Black Tuesday was historically famous because of its huge trading volume and symbolic importance, but Black Monday had the larger one-day percentage decline.