The Dow Jones suffered its largest one-day percentage loss during the 1929 crash on October 28, 1929.
That Monday, the Dow dropped 12.8%. The next day, October 29, brought another severe decline of about 11.7% and became known as Black Tuesday. The two sessions helped turn a major market correction into the defining collapse of the era.
The crash followed years of rising share prices, widespread speculation, and substantial purchases made with borrowed money. When prices began falling, margin calls forced investors to sell, adding pressure to the market. The crash also occurred as the United States economy was moving toward the Great Depression, although the relationship between the market collapse and the depression was complex.
A common mix-up is identifying Black Tuesday as the worst Dow percentage day of the crash. Black Tuesday was more famous, but October 28 produced the larger percentage decline.