On the 4th October 1582, Pope Gregory XIII implemented the Gregorian calendar. Although this calendar is now the most widely used calendar in the world, it was initially only adopted by the Catholic Church and the Papal States since to become a nation’s official calendar it had to be approved by the civil authorities. The only areas to therefore implement it on the specified date were the territories governed by Philip II of Spain, the Polish–Lithuanian Commonwealth, and the Papal States.
Due to a drift between the Julian calendar, the lunar calendar, and the real moon, the date on which the church celebrated Easter had begun to move away from when it had been celebrated by the early church. The Catholic Church disliked this seasonal drift, and so decreed the papal bull Inter gravissimas in early 1582 to reform or – in the words of the Latin text to ‘restore’ – the calendar to align with that at the time of the First Council of Nicaea in 325.
Due to thirteen centuries’ worth of accumulated variations between the existing and new calendars, the change to the Gregorian calendar demanded the deletion of ten days. Consequently, in the territories that adopted the new calendar, the day after the 4th October 1582 became the 15th October – although the day of the week did not change.
Although most Catholic countries swiftly adopted the Gregorian calendar, Protestant governments initially rejected it. However, by the end of the 18th Century most of the countries of Western Europe – including the sizeable British Empire – switched to the Gregorian calendar to ease international trade.