For over 1600 years, most European countries used the Julian calendar which was created by Julius Caesar in 46BC as a reform of the old Roman calendar. While the Julian calendar was not perfect, it proved to be fairly reliable. Over the years, however, its inaccuracies were magnified until it was as many as ten days off the solar year. In a time when agriculture was the backbone of all the economies in the world, it was vital to have a reliable way to reckon the time of year (and thus when crops should be planted). To that end, Pope Gregory XIII issued a papal bull in 1582 to reform the old Julian calendar.
One of the problems that one invariably encounters when trying to make a calendar is the inconvenient fact that solar years cannot be divided into a whole number of days. While we say that the year is 365 days long, the solar year is actually about 365.2425 days. Thus, if one were to create a calendar of only 365 days, one would lost one day in relation to the solar year every four years. That difference would grow over time until the solar year and the calendar year were many days or weeks different from each other. Obviously, this is an undesirable effect, so calendars that are not solar based (like Stonehenge) must include some way of dealing with that extra fourth of a day ever year if they want to remain accurate.
That was what the Julian calendar tried to do. Every four years, the Julian calendar added a day to the end of February to account for that extra fourth of a day. While that usually works, it is not entirely accurate either. The solar year is 365.2425 days long, not 365.25 days long. While that extra .0075 of a day may not seem like very much, after a thousand yeas it is enough to make a difference. It losses about three days every 400 years. By 1582, the Julian calendar was about ten days off of the solar year as a result of many small overcorrections in the use of leap years. What the Gregorian calendar did was change the way that leap years were used. Rather than being used every four years, they were used every four years except for those that were divisible by 100 unless that year was also divisible by 400. The result is to call for 97 leap years ever 400 years instead of 100 leap years ever 400. This calendar is quite accurate and loses only one day every three thousand years.
While a committee appointed by Pope Gregory XIII formulated the necessary reforms rather easily, implementing them was more difficult. Since the calendar had lost ten days over the years, it was necessary to first correct the mistakes in the old calendar before the new calendar could be used. To that end, Pope Gregory issued a Papal Bull in which he removed ten days from October of 1582. Thus, many Europeans went to sleep on October 4, 1582 and woke up on October 15, 1582. While those living in rural villages took little notice of what had happened, some were alarmed that the Pope had "taken ten days out of their lives." Citizens in Frankfurt, Germany and other places rioted in the streets to demonstrate their displeasure.
Another problem was that by the 16th century, the power of the Pope in secular matters had been greatly reduced. The relatively recent creation of new Protestant countries that did not answer to the Pope meant that many countries refused to comply with the Papal Bull. The Pope had no authority, so neither did his calendar, these Protestants reasoned. One by one, however, countries around the world adopted the Gregorian calendar and it is now used in virtually every country in the world. That process took many years, however, so for centuries different countries in Europe reckoned their days differently. It should also be noted that in every case, the government had to strip several days out of the old calendar before adopting the new one. One of the last to give in the "new" calendar was British Empire which did not convert until the mid 18th century. Thus, any dates you see for anything in American history before 1752, might be reckoned in the "Old Style" and might be many days off our current calendar.
Interesting piece thanks