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!