The modern calendar is a compilation of usages dating from the days of Julius Caesar, when the Greek astronomer and mathematician Sosigenes devised for Rome a calendar only essentially familiar to us as our present-day calendar. It was during the late 16th century that work of the 6th century Anglo-Saxon monk, Bede, was submitted to Pope Gregory XIII who accepted the calculations and made the decision to issue a more accurate calendar which ultimately was accepted. The Gregorian calendar was adopted by most Roman Catholic countries. Many protestant countries did not accept the new calendar until the 18th century, including Britain, which meant America as well.
The British finally accepted the Gregorian calendar for itself and all its possessions effective March 25, 1752. But, a remarkable 2-step change was made which is little realized today.
It is most important that the reader understand that the British used to change the year number on March 25th and not January 1st as we do today. On March 24, 1751 the next day would be March 25, 1752 advancing the year as usual. But, a somewhat confusing phenomenon occurred, and for present day imaginations, comprehension requires close attention.
First, it was decreed that 1752 should end with December 31st and not be carried on to the next March 25th.
Second, it was also decreed that the arrival of September 2, 1752 should be called September 14, 1752.
For the sake of clarity we explain; the period of January 1 - March 24, 1751 was the end of an epoch. The year of 1752 began on March 25th and ended with December 31, 1752, thus the earlier days of 1752 never existed, as the deleted days of September 2-13 also never existed. The year 1752 was a very short year; 72 days shorter, in fact.
[Added reference link - McG]