History of the Gregorian calendar{

The Gregorian calendar is the most used today. Under this calendar, a year consists of 365 days or 366 (in a leap year), with a leap day entering the month of February in a leap year. The months of April, June, September, and November have 30 days, while the remaining months have 31 days, except February, which has 28 days in a standard year, and 29 in a leap year.

The Gregorian calendar is a reformed version of the Julian calendar, which itself was a modification of the ancient Roman calendar. The ancient Roman calendar was considered an observational lunar calendar, based on the cycles of the phases of the moon. It was believed that the Romans adopted a 10-month calendar with 304 days, leaving the remaining 50 or more days as unorganized winter.

The Gregorian calendar was introduced on October 4, 1582 by Pope Gregory XIII with the bull Inter gravissimas.

The adoption of the Gregorian calendar has slowed down over the centuries and now dominates as the world's most widely used dating system. In Poland, the calendar was introduced by Stefan Batory immediately after the papal bull was issued. At the same time, it also appeared in Spain, Portugal and almost all of Italy.

The last countries to introduce the Gregorian calendar were Greece, which introduced it in 1923 and Turkey, which introduced it in 1927. Saudi Arabia introduced this calendar in 2016.