The Gregorian calendar, also known as the Western or Christian calendar, was introduced by Pope Gregory XIII by a papal bull on the 24th February 1582. It replaced the Julian calendar which had been the official calendar of Europe since 45 BC, when it was invented by Julius Caesar. The Julian Calendar had 11 months of 30 or 31 days and one month, February, of 28 days, or 29 days on a leap year (every 4th year). Although it was very accurate, only erring from the solar calendar by 11 and a half minutes, by 1582 the Julian calendar was behind the solar calendar by 10 days. Pope Gregory XIII’s reform meant that the calendar would advance by 10 days and included instructions that century years, e.g. 1700 and 1800, would not count as leap years unless they were divisible by 400. The Gregorian calendar, which included a reform of the lunar cycle used by the church, was far more accurate than the Julian calendar as it only differed from the solar calendar by 26 seconds, which only adds up to a difference of 1 day every 3,323 years.
Although the 15th October 1582 was the first day of the Gregorian Calendar, many countries ignored the Papal Bull and carried on using the Julian Calendar. England, for example, did not adopt the Gregorian Calendar until 1752 when the British Calendar Act of 1751 meant that people went to sleep on the night of Wednesday 2nd September 1752 but woke up the next day on Thursday 14th September – confusing!
Ben Snowden, in his article, “The Curious History of the Gregorian Calendar: Eleven Days that Never Were”, points out that the Gregorian Calendar also has its weaknesses:-
“It cannot be divided into equal halves or quarters; the number of days per month is haphazard; and months or even years may begin on any day of the week. Holidays pegged to specific dates may also fall on any day of the week, and vanishingly few Americans can predict when Thanksgiving will occur next year.”
During the French Revolution, there was an attempt to introduce a new calendar, the French Revolutionary Calendar. The Revolutionary Convention decreed on the 5th October 1793 (but computed from the 22nd September 1792) that the year would be divided into 12 months of 30 days and the remaining five days (sans-culottides) were to be feast days. The extra day in leap years was to be added on to the end of the year. Each months was divided into three ten day slots, rather than the usual 7 day week, and every tenth day was to be a day of rest. Although this calendar was simple to understand, it ended along with the Revolution.
- The Curious History of the Gregorian Calendar: Eleven days that never were, Ben Snowden
- The Gregorian Calendar, The Galileo Project
- The Gregorian Calendar, Wikipedia page