Black Monday was the 1987 global crash that prompted the U.S. Federal Reserve to promise liquidity to the financial system.
On October 20, 1987, Federal Reserve Chair Alan Greenspan issued a statement saying the central bank stood ready to serve as a source of liquidity to support the economic and financial system. The statement came after the Dow had fallen 22.6% on October 19, the largest one-day percentage decline in its history.
Central-bank reassurance helped calm fears that market losses would become a broader banking and credit crisis. The Federal Reserve also supplied reserves through the banking system. Markets remained volatile, but the crash did not produce a depression comparable to the 1930s.
This response is sometimes confused with later crisis interventions, especially the extraordinary measures used during 2008. Black Monday refers specifically to the 1987 event, although the same nickname has also been applied to other market collapses in different years.