The Gregorian calendar, used in Europe and in a very large part of the world, takes its name from Pope Gregory XIII who set it up in 1582. This calendar is a correction to the previous calendar, the Julian calendar introduced by Julius Caesar in 46 BC. The starting point of Year 1 is an approximate date of the birth of Jesus.
A time shift in the Julian calendar
Since 45 BC, a calendar of 365 days a year with a leap year every 4 years was used.
This approximation is made in such a way that the duration of a year in the calendar corresponds as precisely as possible to that of a tropical year, i.e. the time that the Sun takes to return to the same position in the sky. During this time, the Sun makes a round trip between its northernmost position and its southernmost position relative to the equator. This leads to the succession of seasons that the calendar wants to reflect.
A tropical year lasts 365.242190 days or 365 days 5 hours 48 minutes 45.2 seconds, so it is not an integer number of days. The approximation provided by the Julian calendar gives a year with an average duration of 365.25 days.
After several centuries, a small difference of 0.0078 days per year began to cause a problem. Thus, because of the imperfection of the Julian calendar, the equinox from spring to the 16th century was shifted to around March 10.
The reform of the calendar by Gregory XIII
In order to put everything in order, Pope Gregory XIII imposed a reform in 1582.
This reform was implemented following the suggestion of several contemporary astronomers and following the instructions of the Council of Trent. It proposes two major corrections. The first is an 11-day jump in the calendar: the day after 4 October 1582 will be 15 October 1582, and 10 days are therefore removed from the calendar. The second is a new way of calculating leap years.
Leap years in the Gregorian calendar
In the Gregorian calendar, the tropical year was approximated to 365.2425 days. To give a whole number of days to the year, we add, as in the Julian calendar, a leap day in February:
To all years that are divisible by 4,
unless these years are divisible by 100, in which case they are not leap years.
However, if the year is also divisible by 400, it will be a leap year.
Therefore, the years 1700, 1800, 1900, 2100, 2200 and 2300 are not leap years, but the years 1600, 2000 and 2400 are leap years.
In summary, there are 97 leap years every 400 years. The first rule alone provides an average year of 365.24 days every 100 years. The second rule allows us to get very close to the tropical year, i.e. 365.2425 days every 400 years. It is only after 3000 years that the time shift between the mean Gregorian year and the tropical year amounts to one day.
Adoption of the Gregorian calendar
October 15, 1582, was the official date of the introduction of the Gregorian calendar.
However, not all countries immediately adopted this calendar. Russia waited until 1917, which is why the October revolution is still commemorated in November. Sweden became so confused in the 18th century that it had to give 30 days in February in 1712. Only in 1923 did Greece adopt the Gregorian calendar. Currently, the majority of countries have adopted the Gregorian calendar as their official calendar, with the exception of Afghanistan, Ethiopia, and Iran.
Concerning Belgium, some sources mention that the regions corresponding to the present southern Belgium today have switched to the Gregorian calendar from 1582 to 1583. In some regions (Flanders and Hainaut), Christmas Day was not celebrated in 1582 as December 21, 1582, was immediately followed by January 1, 1583. Even if Brabant and Zeeland had made the transition earlier (14–25 December), it was not without difficulties. The Prince-Bishopric of Liège introduced the Gregorian reform in February 1583.
You can find more information about calendars on, for example, The Calendar FAQ.