The 1929 Wall Street Crash's largest one-day percentage fall occurred on October 28, 1929, known as Black Monday.
The Dow Jones Industrial Average fell 12.82% that day. The next session, October 29, brought another severe decline and became known as Black Tuesday. These two days are often grouped together when people describe the climax of the 1929 crash.
The collapse followed years of rising share prices, widespread speculation, and purchases made with borrowed money. Selling pressure intensified in October, while banks and investors struggled to support inflated prices. The crash did not by itself cause the entire Great Depression, but it severely damaged confidence and helped deepen the economic downturn.
A common mix-up is to identify October 24, Black Thursday, as the biggest percentage fall. That day was dramatic, but Black Monday recorded the larger single-day percentage decline.