In the financial industry, Black Monday refers to the devastating stock market crash that began on Monday, May 14, 1979. While economic indicators had been poor for several months in the spring of 1979, the severity of the crash was unexpected by many traders, especially since the market had seen gains on the 11th prior to its close.
The crash was caused by a variety of factors - the May 5th announcement of a merger between US Bank and First Bank of New York, the decline in housing values in California and Texas since late 1978, and a slowdown in speculative bubbles in a variety of fields including mortgages, construction companies, and other "light commodities." Also, due to the splitting of the stocks of multiple major companies during the late 1970's by many major companies, individual stocks had low individual worth and were volatile, exacerbating the crisis.
On May 14, the US stock market lost 18.28% of its value in a single day. The DJIA, which had opened at 3,012 that morning, fell to 2,355 at the close. By Friday, May 18, the DJIA had dropped to 2,005, having lost a third of its value in a single week. This would end one of the most volatile up-and-down trading weeks in history, and the following three months would see an even further precipitous decline. The DJIA would halve its value from its April 27 peak of 3,259 by August 3 and by September 4, it had lost almost 80% of its value as investors sold stocks at an unprecedented rate and made the infamous "Run on the Bank" in June.