black Friday

from the GNU version of the Collaborative International Dictionary of English.

• Any Friday on which a public disaster has occurred, as: In England, December 6, 1745, when the news of the landing of the Pretender reached London, or May 11, 1866, when a financial panic commenced. In the United States, September 24, 1869, and September 18, 1873, on which financial panics began.