The invention of the calendar (2,500 BC)

For a people in the process of civilization, it is essential to divide time in order to fix the major events of the community. In  the third millennium BC, the cities of Babylon were the first to apply a  calendar, corresponding to the movements of the Moon, to which they  added if necessary additional months to maintain a correspondence with  the seasons of the year

The Roman calendar

The  first Roman calendar, introduced towards the VIIth century BC,  separated in 10months a year of 304 days which began in March. The  months of January and February were added later, but it was necessary  to insert another month about every other year, because the months were  only 29 or 30 days. The  days were designated by a method of counting backwards from three  pivotal dates: the calends at the beginning of the month, the ideas in  the middle and the nones, which fell on the ninth day before the ideas. This  calendar became hopelessly confused when the Roman rulers, who were  returned to fix the days and months to add, abused their authority to  extend their term or change the election date.

In 46 BC, Julius Caesar decided, on the advice of the Greek astronomer Sosigene, to establish a new calendar. This  calendar, known as the Julian calendar, fixed the duration of a normal  year at 365 days and that of a leap year, every 4 years, at 366 days -  the redoubled day being that of February 24th. Caesar also brought back the beginning of the year to January 1 (instead of 1 March).

the Gregorian calendar

The Julian calendar lasted 11 minutes and 14 seconds longer than the solar year. This difference accumulated so much that in 1582 the vernal equinox (see Ecliptic) fell 10 days before the calendar date. To make the equinox fall around March 21st, as in 325 AD. AD  (year of the first council of Nicea, which had established the main  rules of ecclesiastical computation), Pope Gregory XIII decreed that 10  days, that year, were to be removed from the calendar. He  instituted a new calendar, called since Gregorian calendar, removing  all secular leap years, with the exception of those whose vintage was  divisible by 400. Thus, 1600 was a leap year, but 1700 and 1800 were  normal years.

The Gregorian calendar was slowly extended to all Europe. Nowadays, it is used in most of the Western world, as well as in some Asian countries.