Decimal Day began on 15 February 1971, when the United Kingdom adopted decimal coinage.
Before the change, one pound equaled 20 shillings and 240 pence. Decimal Day replaced that complicated system with 100 new pence to the pound. The transition required new coins, public education, changes to prices, and adjustments for businesses and cash registers.
The United Kingdom introduced the 5p and 10p coins before the changeover, followed by the 50p coin in 1969. On Decimal Day, the halfpenny, penny, and two-pence coins entered circulation, while several old coins were withdrawn or later demonetized.
The phrase is sometimes incorrectly applied to the introduction of the euro, but Decimal Day concerned Britain’s own pound. Ireland decimalized its currency on 15 February 1971 as well, although it was a separate national change involving the Irish pound.