The 1987 market crash associated with the Dow losing 508 points in one session was Black Monday.
Black Monday occurred on 19 October 1987, when the Dow Jones Industrial Average fell 508 points, or 22.6%. The decline was the largest one-day percentage loss in the Dow’s history and was mirrored by severe falls in stock markets around the world.
The crash followed a sharp rise in US share prices and growing anxiety about interest rates, trade imbalances, and the dollar. Program trading and portfolio-insurance strategies were widely discussed because automated or rules-based selling could reinforce falling prices.
Black Monday is not the same as Black Tuesday in 1929. The two names refer to different crashes, separated by nearly six decades, and the 1987 event did not produce a depression comparable to the 1930s.