Under the Julian calendar, which was based on the solar cycle, the year was divided into 12 months of 365 days. An extra day was added every fourth year. This resulted in a year having on average 365 ¼ days.
The Julian calendar was used in all the countries governed by the Romans. This included England and much of Europe. It continued to be used (and maintained) by the Roman Catholic Church, which inherited many Roman institutions after the downfall of the Roman Empire. As a result, the Julian calendar was in widespread use across much of Europe for many centuries.
The Julian calendar assumes the year is exactly 365.25 days long. Unfortunately, the actual solar year is slightly shorter (it is 365.242199 days to be exact). Although the difference appears minor, it can add up over the centuries. In fact, every 129 years, the Julian calendar slipped one additional day out of synchronization with the actual solar year.
This caused a problem within the Roman Catholic Church, who came to realize in the 1500’s that their reliance on the Julian calendar was causing them to incorrectly calculate the date of the spring equinox (the spring equinox is the one day in spring when there is exactly 12 hours of sunlight and 12 hours of darkness). Easter, one of the most sacred days in the Christian religion, is calculated from the spring equinox (Easter is the first Sunday following the full moon after the spring equinox).
To make matters worse, many other Christian observances (such as Lent, for example) are determined from the date of Easter. Therefore, if Easter was calculated incorrectly, then many other religious observances would be celebrated on the wrong day. This caused considerable controversy within the Catholic Church and resulted in several commissions to try to find a solution. It cumulated with Pope Gregory XIII, who in 1582 issued a papal bull that resulted in several calendar revisions, the most important being:
• It established what is now known as the Gregorian calendar (named after Pope Gregory XIII).
• The new Gregorian calendar had an extra day in those years that were divisible by 4 (just like the old Julian calendar), but unlike the Julian calendar, it did not add an additional day in years that were divisible by 100, unless the year was also divisible by 400. Thus, under the Gregorian calendar, the years 1700, 1800 and 1900 were not leap years, but the years 1600 and 2000 were leap years.
• To make up for the errors in the old Julian calendar, ten days were omitted from the new Gregorian calendar. Thus, Thursday, 4 October 1582 in the old Julian calendar was immediately followed by Friday, 15 October 1582 in the new Gregorian calendar.