The largest single-day percentage fall in the Dow before 1987, recorded on October 28, 1929, was called Black Monday.
On October 28, the Dow Jones Industrial Average dropped 12.82%. The fall came after a series of unstable sessions in late October and was followed by another severe decline on October 29, commonly called Black Tuesday. Together, these days became central episodes in the Wall Street Crash of 1929.
The crash followed years of strong speculation, widespread margin buying, and rapidly rising share prices. When confidence weakened, forced selling and fear intensified the decline. The crash did not by itself create every feature of the Great Depression, but it helped undermine confidence and exposed weaknesses in the financial system.
A common mix-up is treating Black Monday as the single name for the entire 1929 crash. The term specifically identifies October 28 in this context, while Black Tuesday identifies October 29. The later 1987 crash also occurred on a Black Monday, making the dates important to distinguish.