On this day exactly fifty years ago, 15th February 1971, Britain finally got rid of its archaic monetary system of pounds, shillings, and pence, and decimalised its coinage. Previously there were twelve pennies to a shilling, and twenty shillings to a pound (or 240 pennies to a pound, if you like). New coins (initially called new pennies, but quickly known in the vernacular simply as pees) were introduced, with 100 new pence making up the pound. The pound itself didn't change in value, as each new pence was worth 2.4 old pennies.
It was obviously well flagged beforehand, with the term D-Day being used in TV, radio, and newspaper public service advertisements, more than a year in advance. Many, particularly of the older generation, were completely opposed to the change. But, within weeks of the changeover, just about everyone had got used to the new coinage, and life went on as normal (with the exception that children at school found adding and subtracting currency a whole lot easier).
Apparently they choose a day in February for the changeover, as that month was traditionally a quiet month financially. And they chose the 15th, as a number of years earlier both South Africa and Australia had decimalised their currencies on Valentines Day, and the Brits didn't want to be seen to be totally following in the footsteps of its former colonies.
Because of the close links between the British Pound and the then Irish Punt, Ireland also decimalised its currency on the same day.