The Gregorian calendar is the most widely used international calendar. It is called after Pope Gregory XIII who introduced the calendar in 1582. The Gregorian calendar replaced the Julian calendar, at first in countries that were predominantly Roman Catholic. It was introduced in other countries in the ensuing centuries.
The Julian calendar was instituted by the Roman Emperor Julius Caesar in 46 BC. It calculated the year at 365 days and 6 hours. However the actual length of the year is 365 days, five hours, forty-eight minutes, forty six seconds.
During the centuries following the introduction of the Julian calendar the discrepancy led to a slippage of days. Easter for instance was being celebrated later each year. This led Pope Gregory to introduce the new calendar in 1582. It was not introduced in Great Britain and Ireland until 1752.
By the time it was introduced in Great Britain and Ireland on September 2nd 1752 in the Julian calendar there was a difference of 12 days. The introduction of the Gregorian calendar meant that people went to bed on September 2nd and woke the following morning on September 14th. It is reported that people protested at the loss of 12 days from their lives. The Gregorian was not introduced in Greece until 1924, Iran 1925 and Turkey 1926.
The Gregorian calendar was instituted by Pope Gregory XIII by the papal bull ‘Inter gravissimas’ in the year 1582 On This Day.