The 1929 date associated with the Wall Street crash’s largest one-day percentage fall in the Dow was Black Monday.
On October 28, 1929, the Dow Jones Industrial Average fell 12.82 percent. The next day, October 29, brought another severe decline of 11.73 percent and became known as Black Tuesday. These two sessions followed Black Thursday, October 24, when heavy selling first drew widespread public attention.
The crash followed years of rising U.S. share prices, extensive margin buying, and growing economic imbalances. The collapse did not by itself cause every feature of the Great Depression, but it became its most famous financial symbol. A common mix-up is to call October 29 the biggest percentage-loss day; October 28 was larger by percentage, while October 29 involved exceptionally heavy trading volume.
The Dow did not regain its September 1929 peak until 1954.