Source : Webster's Revised Unabridged Dictionary (1913)

Black Friday \Black Friday\
   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.