The Change From Julian to Gregorian Calendar
The Gregorian calendar, which is the calendar used today, was first introduced by Pope Gregory XIII via a papal bull in February 1582 to correct an error in the old Julian calendar.
This error had been accumulating over hundreds of years so that every 128 years the calendar was out of sync with the equinoxes and solstices by one additional day.
As the centuries passed, the Julian Calendar became more inaccurate. Because the calendar was incorrectly determining the date of Easter, Pope Gregory XIII reformed the calendar to match the solar year so that Easter would once again “fall upon the first Sunday after the first full moon on or after the Vernal Equinox.”
Ten days were omitted from the calendar to bring the calendar back in line with the solstices, and Pope Gregory XIII decreed that the day following Thursday, October 4, 1582 would be Friday, October 15, 1582 and from then on the reformed Gregorian calendar would be used.
The Catholic countries of Italy, Poland, Portugal, and Spain immediately observed the calendar change, but for almost two hundred years Protestant countries refused to change to the new calendar because it had reformed by a Catholic Pope. The Greek Orthodox countries didn’t make the change until the start of the 20th century.
In many countries the Julian Calendar was used by the general population long after the official introduction of the Gregorian Calendar. Events were recorded in the 16th to 18th Centuries with various dates, depending on which calendar was used. Dates recorded in the Julian Calendar were marked “O.S.” for “Old Style”, and those in the Gregorian Calendar were marked “N.S.” for ”New Style”.
New Year’s Day had been celebrated on March 25 under the Julian calendar in Great Britain and its colonies, but with the introduction of the Gregorian Calendar in 1752, New Year’s Day was now observed on January 1. When New Year’s Day was celebrated on March 25th, March 24 of one year was followed by March 25 of the following year. When the Gregorian calendar reform changed New Year’s Day from March 25 to January 1, the year of George Washington’s birth, because it took place in February, changed from 1731 to 1732. In the Julian Calendar his birthdate is Feb 11, 1731 and in the Gregorian Calendar it is Feb 22, 1732. Double dating was used in Great Britain and its colonies including America to clarify dates occurring between 1 January and 24 March on the years between 1582, the date of the original introduction of the Gregorian calendar, and 1752, when Great Britain adopted the calendar.
Double dates were identified with a slash mark (/) representing the Old and New Style calendars, e. g., 1731/1732.