The Gregorian calendar was introduced on October 4, 1582, in accordance with the bull of February 24, 1582 of Pope Gregory XIII, whose name it still bears today.
It corrects the ancient Roman Julian calendar, making some corrections to more accurately account for the length of the tropical year. In the Julian calendar, the time between two consecutive vernal equinoxes is assumed to be 365.25 days and the fractional part is compensated, with one day more every four years (leap year). In the Gregorian calendar, years that are multiples of 100 are not leap years (except for years that are multiples of 400, i.e., three leap years are skipped every four centuries, such as 1700, 1800, and 1900), which gives an average year length of 365.2425 days. This value is very close to the length of the tropical year - for example, the time between the vernal equinoxes in 2000 was 365.242374 days.
The Gregorian calendar was adopted in different countries at different times, and this process lasted almost three and a half centuries. It was introduced in Bulgaria by a law voted by the National Assembly on March 14, 1916, with the counting of time only according to the new style starting from April 1, 1916 according to the Julian calendar (April 14 according to the Gregorian calendar).
This is necessary due to the involvement of our homeland in the First World War.
Saudi Arabia switched to the Gregorian calendar as of October 2, 2016 as the basis for the payment of salaries to civil servants.
When the kingdom was founded in 1932, the Islamic lunar calendar was adopted. The Islamic year is on average 11 days shorter than the Gregorian one.