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1582 Pope Gregory XIII issued a bull on the transition to the Gregorian calendar
On February 24, 1582, Pope Gregory XIII issued a bull about the transition to a new, astronomically more faithful calendar, which was called "Gregorian", in contrast to the former, "Julian", introduced as early as 45 BC by Julius Caesar. By the XVI century, the Julian calendar, due to lack of accuracy, was greatly divorced from reality: thus, the day of the spring equinox, which was the most important for the determination of the Easter date, moved to March 11 from March 21. Prior to Gregory XIII, the project was attempted by Pope Paul III and Pius IV, but they did not succeed. The preparation of the reform on the orders of Gregory XIII was carried out by astronomers Christopher Clavius ​​and Luigi Lilio (also Aloisius Lily). The results of their work were recorded in the papal bull, named after the first line of Inter gravissimas ("Among the most important").
The new calendar came into force on October 4, 1582. And immediately at the time of adoption, he shifted the current date by 10 days due to accumulated errors. In it began to operate a new, more precise rule about the leap year. Henceforth, the year began to be considered a leap year, that is, to contain 366 days, if its number is divided into 4 without divide and is not divided by 100, or its number is divided without a remainder by 400. In addition, the rules for calculating Christian Passover have been modified.
Thus, over time, the Julian and Gregorian calendars diverge more and more: for 1 day per century, if the number of the previous century is not divisible by 4. The Gregorian calendar reflects the true state of things much more accurately than the Julian calendar. It gives a much better approximation to the tropical year.
The instruction to switch to the new calendar was immediately accepted for execution only by Catholic countries: Italy, Spain, Portugal, France. German principalities moved to a new style in 1700, England - in 1752, Russia - in 1918, Yugoslavia and Greece - in 1925, and Vietnam - only in 1967.