In the year 1572, Ugo Boncompagni became Pope Gregory XIII and there was a crisis of the calendar - one of Christianity's most important dates was falling behind with respect to the seasons. Easter, which is based on the date of the vernal equinox (the first day of Spring), was being celebrated too early in the month of March. The cause of this calendrical confusion was the over 1,600 year-old Julian calendar, established by Julius Caesar in the year 46 BCE.

Julius Caesar took control over the chaotic Roman calendar, which was being exploited by politicians and others with the haphazard addition of days or months. It was a calendar horribly out-of-synch with the seasons of the earth, which are the result of the rotation of the earth around the sun. Caesar developed a new calendar of 364 1/4 days, closely approximating the length of the tropical year (the time it takes the earth to go around the sun from the beginning of spring to the beginning of spring). Caesar's calendar was normally 365 days long but included an extra day (a leap day) every four years to account for the extra one-quarter of a day. The intercalary (inserted into the calendar) day was added prior to February 25 each year.

Unfortunately, while Caesar's calendar was almost accurate, it wasn't quite accurate enough because the tropical year is not 365 days and 6 hours (365.25 days), but is approximately 365 days 5 hours 48 minutes, and 46 seconds (365.242199 days). Therefore, the calendar of Julius Caesar was 11 minutes and 14 seconds too slow. This added up to be a full day off every 128 years.

While it took from 46 BCE to 8 CE to get Caesar's calendar functioning properly (initially leap years were being celebrated every three years instead of every four), by the time of Pope Gregory XIII the one day every 128 years added up to a full ten days of error in the calendar. (Purely by luck did the Julian calendar happen to celebrate leap years on years divisible by four - during Caesar's time, the numbered years of today didn't exist).

A serious change needed to take place and Pope Gregory XIII decided to repair the calendar. Gregory was aided by astronomers in developing a calendar that would be more accurate than the Julian calendar. The solution they developed was almost perfect.

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The new Gregorian calendar would continue to be comprised of 365 days with an intercalary added every four years (moved to after February 28 to make things easier) but there would be no leap year in years ending in "00" unless those years were divisible by 400. Therefore, the years 1700, 1800, 1900, and 2100 would not be a leap year but the years 1600 and 2000 would. This change was so accurate that today, scientists need only add leap seconds every few years to the clock in order to keep the calendar matching the tropical year.

Pope Gregory XIII issued a papal bull, "Inter Gravissimus" on February 24, 1582 that established the Gregorian calendar as the new and official calendar of the Catholic world. Since the Julian calendar had fallen ten days behind over the centuries, Pope Gregory XIII designated that October 4, 1582 would be officially followed by October 15, 1582. The news of the calendar change was disseminated across Europe. Not only would the new calendar be utilized but ten days would be "lost" forever, the new year would now begin on January 1 instead of March 25, and there would be a new method of determining the date of Easter.

Only a few countries were ready or willing to change to the new calendar in 1582. It was adopted that year in Italy, Luxembourg, Portugal, Spain, and France. The Pope was forced to issue a reminder on November 7 to nations that they should change their calendars and many did not heed the call. Had the calendar change been promulgated a century earlier, more countries would have been under Catholic rule and would have heeded the Pope's command. By 1582, Protestantism had spread across the continent and politics and religion were in disarray; additionally, the Eastern Orthodox Christian countries would not change for many years.

Other countries later joined the fray over the following centuries. Roman Catholic Germany, Belgium, and the Netherlands switched by 1584; Hungary changed in 1587; Denmark and Protestant Germany switched by 1704; Great Britain and its colonies changed in 1752; Sweden changed in 1753; Japan changed in 1873 as part of Meiji's Westernization; Egypt changed in 1875; Albania, Bulgaria, Estonia, Latvia, Lithuania, Romania, and Turkey all changed between 1912 and 1917; the Soviet Union changed in 1919; Greece switched to the Gregorian calendar in 1928; and finally, China changed to the Gregorian calendar after their revolution of 1949!

Change wasn't always easy, however. In Frankfurt as well as London, people rioted over the loss of days in their lives. With each change to the calendar around the world, laws established that people could not be taxed, paid, nor would interest accrue over the "missing" days. It was decreed that deadlines still had to take place in the correct number of "natural days" following the transition.

In Great Britain, Parliament legislated the change to the Gregorian calendar (by this time simply called the New Style calendar) in 1751 after two unsuccessful attempts at change in 1645 and 1699. They decreed that September 2, 1752 would be followed by September 14, 1752. Britain needed to add eleven days instead of ten because by the time Britain changed, the Julian calendar was eleven days off the Gregorian calendar and tropic year. This 1752 change also applied to the American colonies of Britain so the change was made in the pre-United States and pre-Canada at that time. Alaska didn't change calendars until 1867, when it transferred from a Russian territory to a part of the United States.

In the era after the change, dates were written with O.S. (Old Style) or N.S. (New Style) following the day so people examining records could understand whether they were looking at a Julian date or a Gregorian date. While George Washington was born on February 11, 1731 (O.S.), his birthday became February 22, 1732 (N.S.) under the Gregorian calendar. The change in the year of his birth was due to the change of when the change of the new year was acknowledged. Recall that prior to the Gregorian calendar, March 25 was the new year but once the new calendar was implemented, it became January 1. Therefore, since Washington was born between January 1 and March 25, the year of his birth became one year later upon the switch to the Gregorian calendar. (Prior to the 14th century, the new year change took place on December 25.)

Today, we rely on the Gregorian calendar to keep us almost perfectly in line with the rotation of the earth around the sun. Imagine the disruption to our daily lives if a new calendar change were required in this most modern era!