The October 1929 day when heavy selling began before Black Tuesday was Black Thursday, October 24, 1929.
The Dow Jones Industrial Average fell sharply that morning as investors rushed to sell shares. A group of major bankers met at J. P. Morgan & Co. and reportedly bought leading stocks to support prices, producing a temporary rebound.
Selling returned the following week. Black Monday occurred on October 28, and Black Tuesday followed on October 29, when about 16 million shares changed hands and prices collapsed further.
Black Thursday is therefore part of the sequence of 1929 crash days rather than the entire crash itself. The crash continued over subsequent weeks, and the wider economic effects contributed to the Great Depression, although the Depression had multiple causes.