Also A day of economic catastrophe, as in We feared there'd be another Black Friday. This usage dates from September 24, 1869, a Friday when stock manipulators Jay Gould and James Fisk tried to corner the gold market and caused its collapse. The adjective black has been appended to similar occasions ever since, including October 29, 1929, the Tuesday of the market collapse that marked the start of the Great Depression, and Black Monday of October 19, 1987, when the stock market experienced its greatest fall since the Great Depression.
Any day marked by great confusion or activity, as in It was just my luck to be traveling on Black Tuesday. This usage, too, is based on the events of 1869, marked by economic chaos. It has since been extended to other kinds of confusion, such as an accident hampering traffic during the evening rush hour.