The Gregorian Calendar is the commonly accepted calendar. It was created by Aloysius Lilius, a physician from Naples, and adopted by Pope Gregory XIII during the Council of Trent (1545-1563). Its purpose was to improve accuracy of the currently accepted Julian Calendar, by changing the length of the tropical year from 365.25 days to 365.2425 days. as a result, it would take 3300 years for an error of one day in the tropical year of the Gregorian Calendar.
In the Gregorian Calendar, there are approximately 97 leap years every 400 years. The leaps years are determined by:
As a result, the years 1700, 1800, 1900, 2100, and 2200 are not leap years, but 1600, 2000, and 2400 are leap years.
There is a myth that there are double leap years, with 367 days (With the possible exception of Sweden in 1712).
The astronomer John Herschel (1792-1871) suggested that a better approximation of the tropical years would be 365.24225. This would result in 969 leap years every 4000 years, rather than the accepted 970. This could be achieved by dropping a leap year every 4000 years, hence every year divisible by 4000. However, this practice was never officially established yet.
The Greek Orthodox church practice the Gregorian Calendar slightly differently. They have the rule:
Every year that leaves a remainder of 200 or 600 when divided by 900 is a leap year.
This rule replaces the accepted 400 year rule. As a result, the years 1900, 2100, 2200, 2300, 2500, 2600, 2700, and 2800 are not leap years. This won't create conflict with the rest of the world until the year 2800.
As a result of the rule, the tropical year for the Greek Orthodox is an average of 365.24222 days, more accurate than the accepted Gregorian Calendar's tropical year of 365.2425 days. However, this practice isn't official in the nation of Greece.
In February of 1582, the papal bull declared that the ten days be dropped for the transition from Julian to Gregorian. October 15, 1582 would immediately follow October 4, 1582. The Catholic nations of Italy, Poland, Portugal, and Spain followed this transition, but the Protestant countries were reluctant to change, and the Greek orthodox didn't change until the beginning of the 20th century.
The reason that the 16th and 17th century require the same drop is because the start of the 17th century (1600) is divisible by 400, and thus has the extra leap year at the transition of the century.
Sweden has a curious history involving the transition from Julian to Gregorian. Sweden decided on having a gradual transition by dropping every leap year from 1700 to 1740. As a result, eleven days would be dropped. By the date March 1, 1740, Sweden would be in sync with the Gregorian Calendar, after forty years of being in sync with nobody. In 1700, Sweden omitted the leap year (which was needed in the Gregorian Calendar), but left the leap years in 1704 and 1708 by mistake. As a result, Sweden decided to go back to the Julian Calendar by having a double leap year in 1712. The double leap year resulted in the date February 30 that year. Sweden then made the transition to the Gregorian Calendar in 1753 by dropping eleven days.
December 1912