The Parliament of Catalonia declared independence from Spain in a failed bid for sovereignty on October 27, 2017. The declaration followed an independence referendum, boycotted by pro-union residents, that saw 92% of voters choose independence. The declaration was subsequently declared illegal by Spain's Supreme Court. Many Catalan independence figures such as then-Catalan President Carles Puigdemont were criminally charged as a result of their involvement in the independence attempt. Read the Catalan Declaration of Independence.

On October 27, 1787, the first of the Federalist Papers, written by Alexander Hamilton, John Jay, and James Madison under the pen-name "PUBLIUS" to promote the ratification of the new US Constitution, was published in a New York newspaper.

On October 27, 1986, the so-called "Big Bang" deregulated markets throughout the London Stock Exchange as part of Prime Minister Margaret Thatcher's financial reform program. Learn more about the effects of the Big Bang in the United Kingdom.