In 45 BC, Julius Caesar added another ten days to the existing Roman calendar, and a leap day to February every four years. This became the Julian calendar, with the New Year in January.
Although the Julian calendar was a more accurate calendar than its predecessors, an average Julian year lasted 365.25 days, instead of the actual solar day length of 365.2422, thus resulting in an error of one day every 130 years.
So, Pope Gregory XIII commissioned the Gregorian calendar. As a modification of the Julian calendar, it reduced the average year from 365.25 days to 365.2425 days and adjusted for the drift in the ‘solar’ year that the inaccuracy had caused during the intervening centuries. The Gregorian calendar spaces leap years, to make its average year 365.2425 days long. But it still means it is off by one day every 3236 years!
The Gregorian calendar was adopted in 1582 by most Roman Catholic countries in Europe. But the calendar was met with scepticism from Protestant countries, many of whom believed the calendar was the work of the Devil. Most countries did eventually adopt the calendar as a ‘civil’ international standard; Britain in 1752 and lastly Greece in 1923.