The pope issued the bull (a formal proclamation), known by its first Latin words as Inter Gravissimas, on February 24, 1582. Its aim was to reform the Julian calendar’s method of calculating the date of Easter, which had been standardized in 325 at the First Council of Nicaea, a meeting of Christian bishops that, in turn, had 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 religious 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 holidays fixed to Easter—for example the Pentecost, or Whitsunday, on the seventh Sunday following Easter—might collide with pagan festivals such as the summer solstice.
To rectify this problem, Gregory ordered churches to drop ten days from that year’s calendar, so that Thursday, October 4, 1582, was followed by Friday, October 15. (The pope chose October because it was a month with few holidays or official conflicts in the church.) He also instituted a new system of fewer 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 Catholic 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. Protestant countries such as England and Scotland, which did not recognize the pope’s authority, did not, therefore, immediately adopt the Gregorian calendar. John Dee, a geographer and mathematician who advised, devised a more scientifically exact calendar than Pope Gregory’s, but it never gained widespread use.
In the meantime, most of the Catholic countries, even before accepting the Gregorian reform, dated their new year from January 1. (The church began its ecclesiastical calendar at Christmas, but still celebrated a feast day on January 1.) While remaining on the Julian calendar, Scotland adopted January 1 as the beginning of the new year in 1600. England, by contrast, began its year with the Feast of the Annunciation, known in England as Lady Day and celebrated on March 25. With the Act of Union (1707) joining England and Scotland, these date disparities created a problem. The old ten-day disparity between the Old Style calendar and the seasons had grown, by this time, to eleven days (1700 was a leap year under the Julian, but not the Gregorian, calendar), so that, according to the historian Duncan Steel, a given day might be reflected by as many as three different dates: “February 1, 1720, in Scotland, February 1, 1719, in England, and February 12, 1720, in most of the rest of Europe.”
To acknowledge these differences, many correspondents and publications, including some in Virginia, took to assigning two years—Old Style and New Style—with dates that occurred between January 1 and March 25 in the years after 1582. That practice ceased when Great Britain adopted the Gregorian calendar in 1752. Two years earlier, Parliament had passed “An act for regulating the commencement of the year; and for correcting the calendar now in use.” Known as the Calendar (New Style) Act, it directed that the year 1751 would end on December 31 and the year 1752 would begin on January 1. Additionally, Wednesday, September 2, 1752, should be followed by Thursday, September 14, 1752.