The artifact is a letter signed by Pope Gregory XIII, dated February 9th, 1581, prior to the enactment of the Gregorian calendar which occurred on Thursday, October 4, 1582.
The Gregorian reform consisted of the following:
- Ten days were omitted from the calendar, and it was decreed that the day following (Thursday) October 4, 1582 (which is October 5, 1582, in the old calendar) would thenceforth be known as (Friday) October 15, 1582.
- The rule for leap years was changed. In the Julian calendar a year is a leap year if it is divisible by 4. In the Gregorian calendar a year is a leap year if either (a) it is divisible by 4 but not by 100 or (b) it is divisible by 400. In other words, a year which is divisible by 4 is a leap year unless it is divisible by 100 but not by 400 (in which case it is not a leap year). Thus the years 1600 and 2000 are leap years, but 1700, 1800, 1900 and 2100 are not.
- New rules for the determination of the date of Easter were adopted.
- The position of the extra day in a leap year was moved from the day before February 25th to the day following February 28th.
Pope Gregory XIII
Gregory XIII contributed to the papacy in myriad ways during the twelve years of his reign. Of great significance was the replacement of the inaccurate Julian calendar with what came to be known as the Gregorian System, still in use after five centuries. The new calendar struck ten days in October off the existing calendar thereby giving it the accuracy it needed. Catholic countries followed it immediately and by the 1700’s even England had adopted the calendar.
The calendar was considered to be his most significant accomplishment but Gregory was responsible for so much more. He continued to promote the reforms of the Council of Trent and was influenced by Charles Borromeo in this work. He was the first pope to use a Secretary of State in the modern sense. He appointed a commission of cardinals to oversee the bishops to ensure they resided in their dioceses. He sought to use nunciatures as instruments of reform rather than just as diplomatic missions, and in this spirit he created new ones.