The Dow Jones Industrial Average lost 12.82% on October 28, 1929, the first of two devastating sessions known as Black Monday and Black Tuesday. The fall was the largest percentage decline in the Dow up to that point.
Selling intensified after months of speculation, rising margin debt, and growing concern that share prices had moved far beyond the underlying economy. On Black Monday, the Dow fell 38.33 points, closing at 260.64. The next day, October 29, it dropped another 11.73%, in the session commonly called Black Tuesday.
The 1929 crash did not by itself cause every feature of the Great Depression, but it damaged confidence and exposed serious weaknesses in the financial system. A common mix-up is to attribute the 12.82% figure to Black Tuesday; the percentage belongs to October 28, while Black Tuesday produced the larger point decline.