On 4 October 1582, the Gregorian Calendar reform took effect and the next day was 15 October 1582.
Pope Gregory XIII in his bull Inter Gravisimus on 24 February 1582 issued his decision to implement a calendar reform. He wanted a calendar that was more scientifically accurate and which placed the celebration of Easter more closely to the vernal equinox from which it had been drifting ever since the Council of Nicea in 325.
The decision to implement the new calendar on October 4 was because there were not as many days dedicated to saints in the period between the 4th and the 15th. Those saints whose days were skipped were celebrated after the 15th.
While most of the Catholic Countries adopted the new calendar, many Protestants objected believing that to accept the Gregorian calendar would be akin to accepting the authority of the papacy. Protestant countries would later use the calculations of Kepler to justify their change to the Gregorian calendar. The Gregorian calendar is still not accepted by many Orthodox Christians who continue to use the Julian calendar.