On this day in 1582, the Gregorian Calendar was adopted by the Catholic Church and the governments of the Catholic areas of Europe. It replaced the Julian Calendar which had been in use in Europe since Roman times. October 4 was the last day of the Julian Calendar, immediately followed by October 15 on the Gregorian Calendar.
Although the Protestant and Orthodox countries of Europe initially rejected the reforms of the Gregrorian Calendar, named after Pope Gregory the XIII, eventually all of Christendom would adopt it. In fact most of the world now uses the Gregorian Calendar as the Standard Calendar for matters of government and economics.
For more on the Gregorian Calendar and other calendar systems, visit the following: