What is Black Wednesday?

Legal Definition
In politics and economics, Black Wednesday refers to 16 September 1992, when the British Conservative government was forced to withdraw the pound sterling from the European Exchange Rate Mechanism (ERM) after it was unable to keep the pound above its agreed lower limit in the ERM. In 1997, the UK Treasury estimated the cost of Black Wednesday at £3.4 billion. In 2005, documents released under the Freedom of Information Act revealed that the actual cost may have been slightly less, £3.3 billion.

The trading losses in August and September were estimated at £800 million, but the main loss to taxpayers arose because devaluation could have made them a profit. The papers show that if the government had maintained $24 billion foreign currency reserves and the pound had fallen by the same amount, the UK would have made a £2.4 billion profit on the pound sterling's devaluation.
-- Wikipedia
Legal Definition
The day September 16, 1992. This day Sterling lost 15% value after it withdrew from the European Union.