The Gregorian calendar, which still bears its name today, was put into use on October 4, 1582. This happened in accordance with a bull from February 24, 1582, issued by Pope Gregory XIII. This calendar represents a significant correction to the ancient Roman Julian calendar.
The main reason for this change was the need to more accurately account for the length of the tropical year. In the Julian calendar, it is assumed that the time between two consecutive spring equinoxes is 365.25 days. To compensate for the fractional part, in the Julian calendar every fourth year is a leap year, containing one day more.
The Gregorian calendar introduces additional clarifications. Years that are multiples of 100 are not leap years, with the exception of those that are multiples of 400. This means that every four centuries, three leap years are skipped, such as 1700, 1800 and 1900. As a result of these corrections, the average length of the year in the Gregorian calendar is 365.2425 days. This value is significantly closer to the length of the tropical year. As an example, the time between the spring equinoxes in 2000 was 365.242374 days.
The adoption of the Gregorian calendar in different countries happened gradually and spread over almost three and a half centuries. In our country, this calendar was introduced by law, voted by the National Assembly on March 14, 1916. The transition to counting time only by the new style began on April 1, 1916, according to the Julian calendar, which corresponds to April 14 according to the Gregorian calendar. This change was caused by Bulgaria's participation in the First World War, one of the bloodiest and most tragic events in the history of Europe.