October 28, 1929, is called Black Monday because the Dow Jones Industrial Average fell 12.8 percent.
The sharp decline came after a weekend in which investors had time to react to worsening market sentiment. On October 28, selling intensified and the Dow lost 38.33 points, equal to about 12.8 percent of its value at the time. The following day, October 29, became known as Black Tuesday.
These sessions were part of the Wall Street Crash of 1929, which continued beyond October. The Dow eventually reached a low in July 1932, having fallen roughly 89 percent from its September 1929 peak.
Black Monday is sometimes confused with October 19, 1987, when markets around the world suffered another famous Black Monday. The 1929 date refers specifically to October 28, while the 1987 date belongs to a different crash.