Throughout the medieval era concerns grew about problems with the Julian calendar. The vernal equinox occurred on later dates each year, which had a knock-effect for the calculation of the date of Easter and other movable feast days. After decades of discussion, on 24th February 1582, Pope Gregory XIII [pictured] issued the papal bull Inter gravissimas, which ordered the 'restoration' of the calendar. The Pope later received a manuscript called Compendiuem novae rationis restituendi kalendarium ("Compendium of the New Plan for the Restitution of the Calendar") from Antonio Lilius, brother to Aloysius, an Italian scholar and author of the treatise, who had died six years previously.Lilius' plan, slightly modified by the German Jesuit scholar Christopher Clavius, required a reduction in the number leap-years. Centennial years (such as 1700 and 1900) would no longer have an extra day unless they were a multiple of 400 (e.g. 1600 and 2000). The reformation also required an adjustment by ten days.
The papal bull required the adoption of the new calendar in all Catholic countries. Consequently, in much of Italy, Spain, Portugal, and the Commonwealth of Polish-Lithuania Thursday 4th October 1582 was followed by Friday 15th October, the first day of what became known as the Gregorian calendar. Poor communications resulted in the Portuguese and Spanish colonies following suit later in the year, as did some Protestant nations.
Over the next two centuries the remaining Protestant nations in Western Europe adopted the reformed calendar, except the Swiss canton of Grisons which held out until 1811. In the twentieth-century the nations of eastern Europe followed suit by which time countries on the other continents had also adopted the Gregorian calendar.
The text of Inter gravissimas is available on the Blue Water Arts site.
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