That pope was Gregory XIII (1502-1585). Gregory instituted a number of reforms in the Catholic Church during his papacy, but his most enduring legacy is undoubtedly his calendar reform. Up until his time, the Catholic Church--and Europeans in general--had used the Julian calendar, named in honor of the Roman general and politician Julius Caesar, who had sponsored it. Unfortunately, the Julian calendar contained an error--it assumed a solar year is exactly 365.25 days long, when in fact it is slightly shorter than that. As a result, an error was introduced into the calculation of the date of Easter, so that after several centuries Easter was being celebrated several days later than it had been in the early church. By the 16th century, the Catholic Church had decided to reform the calendar in order to return the date for Easter to the date on which it had been celebrated in the early church. In 1545, the Council of Trent authorized then Pope Paul III to reform the calendar, but the reform did not actually take place until Gregory's papacy.
The reformed calendar was introduced in October 1582--the Julian calendar day October 4, 1582 was followed by Gregorian calendar day October 15, 1582. The new calendar was quickly adopted by Catholic countries in Europe, but, perhaps not surprisingly, given the religious hostility of the time, Protestant countries were reluctant to accept the new calendar. In fact, Great Britain and its American colonies would not adopt the Gregorian calendar until 1752--two decades after Washington's birth. Consequently, as a result of Britain's (late) adoption of the Gregorian calendar, the date of Washington's birth was moved back by a year and eleven days.
The public celebration of Washington's birthday--on February 22--began during Washington's own lifetime, but Washington's birthday did not become an official Federal holiday until 1879, according to the National Archives. In 1968, the Monday Holiday Act moved the official celebration of Washington's birthday to the third Monday in February, which means, ironically--as the National Archives points out--it never falls on the actual date of Washington's birth. However, contrary to what most seem to believe, Washington's birthday has never been officially designated "Presidents' Day"! However, from my own experience, protesting against this common misconception is a lost cause.
Image: Gilbert Stuart's portrait George Washington, from Wikimedia Commons
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