The day after Thanksgiving Day, generally regarded as the first day of the Christmas season, and the busiest shopping day of the year. (Although this usage may also very possibly originate in Etymology 1)
Day (Sept. 24, 1869) when plunging gold prices precipitated a U.S. stock-market panic. An attempt by Jay Gould and James Fisk to corner the market in gold and drive up its price depended on preventing the sale of government gold, an arrangement assured through the two men's political influence. When Pres. Ulysses S. Grant heard of the scheme, he ordered the government to sell $4 million in gold, which caused the price to drop and produced a panic selling of other stocks
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