The Gregorian Calendar
Today the Gregorian calendar was instituted by Pope Gregory XIII in 1582 and was adopted by much of Christendom, and of course was ignored by the apostate countries of the Protestant revolt. This reformed calendar altered the Julian, or Old Style, a system of leap years and, by removing ten days from October 1582, adjusted the timing of the Easter observance so that it better coincided with the spring season. Many of the countries that adopted the Gregorian calendar had already recognized 1 January as the beginning of the new year. Until it adopted the reformed calendar in 1752, apostate England dated its new year at March 25, or the observance of Lady Day (the Feast of the Annunciation). As a result, many English correspondence and publications, including some in Virginia, marked those dates between January 1 and March 25, during the years 1582–1752, with two years: Old Style and New Style.
By issuing the Bull Inter Gravissimas, on 24 February 1582 Pope Gregory XIII’s aim was to reform the Julian calendar’s method of calculating the date of Easter, which had been standardised in 325 at the First Council of Nicaea, which largely followed precepts laid down at the First Council of Arles (314). These early councils established rules to set the date of the moveable feast of Easter based on an assumption that the solar year lasts 365.25 days, an overestimation of about eleven minutes. This astronomical mistake had liturgical implications because it meant that over time the calendar and the seasons became misaligned. As Easter came later in the year at a rate of about one day every 130 years, the Pope worried that other holy days fixed to Easter such as Pentecost, or Whitsunday.
To rectify this problem, Gregory ordered that dioceses throughout the Catholic world to drop ten days from that year’s calendar, so that Thursday, 4 October 1582, was followed by Friday, 15 October. (The Pope chose October because it was a month with no holy days.) He also instituted a new system with a reduced number of leap, or extra, days. Rather than adding a day to the month of February every four years, as the Julian calendar did, the system would add a leap day only in those years whose numbers can be evenly divided by 4, the exception being those years that are also divisible by 100, unless they are also divisible by 400 (for example, the year 1600).
Much of Christendom in Europe quickly aligned its calendars according to Gregory’s decree. Spain, Portugal, Poland, and most of Italy did so in October 1582; France and the Spanish Netherlands in December 1582; Austria in 1583; Bohemia and Moravia in January 1584; Hungary in 1587; and, a bit later, Prussia in 1610. The apostate countries such as England and Scotland, which did not recognize the papal authority, did not, therefore, immediately adopt the Gregorian calendar.