[Extracted from the Internet]
Calendars were devised to reckon the past or future time, to show how many days were there until a certain event - the harvest or a religious festival - takes place or how long since something important happened. In colder countries, the concept of the year was determined by the seasons, specifically by the end of winter. But in warmer countries, where the seasons are less pronounced, the Moon became the basic unit for time reckoning.
The Egyptian Calendar
The ancient Egyptians used a calendar with 12 months of 30 days each, for a total of 360 days per year. The Egyptians had calculated that the solar year was actually closer to 3651/4 days, and about 4000 B.C. they added five extra days at the end of every year to bring it more into line with the solar year. These five days became a festival because it was thought to be unlucky to work during that time. As a correction for the 1/4 day was not made, as the years passed, the Egyptian months fell out of sync with the seasons, so that the summer months eventually fell during winter.
The Roman Calendar
When Rome emerged as a world power, the difficulties of making a calendar were well known, but the Romans complicated their lives because of their superstition that even numbers were unlucky. Hence their months were 29 or 31 days long, with the exception of February, which had 28 days. However, four months of 31 days, seven months of 29 days, and one month of 28 days added up to only 355 days. Therefore the Romans invented an extra month called Mercedonius of 22 or 23 days. It was added every second year.
The Julian Calendar
Even with Mercedonius, the Roman calendar eventually became so far off that Julius Caesar, advised by the astronomer Sosigenes, ordered a sweeping reform in 45 B.C. One year, made 445 days long by imperial decree, brought the calendar back in step with the seasons. Then the solar year (with the value of 365 days and 6 hours) was made the basis of the calendar. The months were 30 or 31 days in length, and to take care of the 6 hours, every fourth year was made a 366-day year. Moreover, Caesar decreed the year began with the first of January, not with the vernal equinox in late March.
This calendar was named the Julian calendar. However, despite the correction, the Julian calendar is 111/2 minutes longer than the actual solar year, and after a number of centuries, even 111/2 minutes adds up.
The Gregorian Calendar
By the 15th century the Julian calendar had drifted behind the solar calendar by about a week, so that the vernal equinox was falling around March 12 instead of around March 20. Pope Sixtus IV decided that another reform was needed and called the German astronomer Regiomontanus to Rome to advise him in 1475, but unfortunately he died shortly afterward. The pope's plans for reform died with him.
Then in 1545, the Council of Trent authorized Pope Paul III to reform the calendar once more. Most of the mathematical and astronomical work was done by Father Christopher Clavius, S.J. The immediate correction, advised by Father Clavius and ordered by Pope Gregory XIII, was that Thursday, Oct. 4, 1582, was to be the last day of the Julian calendar. The next day would be Friday, Oct. 15.
For long-range accuracy, a formula suggested by the Vatican librarian Aloysius Giglio was adopted: every fourth year is a leap year unless it is a century year like 1700 or 1800. Century years can be leap years only when they are divisible by 400 (e.g., 1600 and 2000). This rule eliminates three leap years in four centuries, making the calendar sufficiently accurate.
For in spite of the revised leap year rule, an average calendar year is still about 26 seconds longer than the Earth's orbital period. But this discrepancy will need 3,323 years to build up to a single day.
The Gregorian reform was not adopted throughout the West immediately. Most Catholic countries quickly changed to the Pope's new calendar in 1582. But Europe's Protestant princes chose to ignore the papal bull and continued with the Julian calendar. It was not until 1700 that the Protestant rulers of Germany and the Netherlands changed to the new calendar. In Great Britain (and its colonies) the shift did not take place until 1752 (September 2, 1752, was a great day in the history of sleep. That Wednesday evening, millions of British subjects in England and the colonies went peacefully to sleep and did not wake up until twelve days later on Thursday the fourteenth), and in Russia a revolution was needed to introduce the Gregorian calendar in 1918. In Turkey, the Islamic calendar was used until 1926.
April Fool's Day
April Fool's Day is known for practical joking and celebrated on the first of April. Prior to the adoption of the Gregorian calendar in 1582, the date 1st April was observed as New Year's Day by many cultures. The holiday closely followed the festival of the vernal equinox (March 20th or 21st). Sri Lanka and other countries in the region still celebrate New year in April (with the 12 day correction). The new Gregorian calendar introduced by Pope Gregory XIII in 1582 called for New Year's Day to be celebrated Jan. 1. Many people either refused to accept the new date, or did not learn about it, and continued to celebrate New Year's Day on April 1. Those who refused or forgot were ridiculed by being sent foolish gifts and invitations to nonexistent parties.
The English gave April Fool's Day its first widespread celebration during the 18th century.