The Gregorian Calendar gets its name from Pope Gregory XIII, who in 1582 made changes to the older Julian calendar. The old system’s March 21st had drifted 10 days from the true vernal equinox, which was part of the calculation of Easter, so in February 1582 a papal bull was issued declaring that the 4th of October would be followed by the 15th of October and that there would be new rules for leap days. In the new system, the leap days would be added on February 29th every four years, but century years not evenly divisible by 400, such as 1700, 1800 and 1900 would not be leap years. However, the years 1600 and 2000 would be.
The adoption of the new calendar system is the start of a lot of confusion. Because the Pope issued the bull making the changes, many Protestant countries did not adopt the new calendar system. By the 1750s, Great Britain and its territories (including the colonies that later became the United States) still had not adopted the new calendar. But it was decided that in 1752 the Gregorian calendar would finally be adopted. Because the Julian calendar still had a leap day in 1700, the correction would now have to be 11 days to bring it into line with the Gregorian system. So, September 2nd 1752 was followed by September 14th.