Decimal Day was Monday 15 February 1971, when the United Kingdom and Ireland decimalised their £sd currencies of pounds, shillings, and pence.
An introductory pack of the new currency
Along with a few other non-circulating coins, sovereigns (including half, double and quintuple sovereigns) survived decimalisation, and remain legal tender to date, though they have far greater worth as bullion coins and as numismatic collectibles than their face value.