The Dow Jones Industrial Average lost 12.8% on Black Monday, October 28, 1929.
The decline was measured as a 38.33-point fall, taking the Dow from 298.97 to 260.64. Investors facing margin calls rushed to sell, and the selling overwhelmed the earlier efforts by bankers to steady prices after Black Thursday, October 24.
Black Monday was followed by Black Tuesday, October 29, when the Dow fell another 11.73%. Together, the two sessions produced a drop of roughly 23%, although the crash continued after those dramatic days. The Dow reached an interim low in November 1929 before staging a temporary rebound.
A common mix-up is confusing 1929's Black Monday with Black Monday on October 19, 1987, when the Dow fell 22.6% in one day. October 28, 1929, was the largest single-day percentage decline of that 1929 crash, while October 29 is remembered as Black Tuesday. The collapse helped destroy confidence and contributed to the economic contraction that became the Great Depression.