this is full history of calendar and how it started please read on
The ancient Egyptians were the first to measure a year with any exactness. They knew that the best time to plant was right after the Nile River overflowed each year. Their priest noticed that between each overflowing the moon rose 12 times. So they counted 12 months or months and figured out when the Nile would rise again.
But it still wasn’t exact enough. At last the Egyptian priest noticed that each year, about the time of the flood, a certain bright star would rise just before the sun rose. They counted the days that passed before this happened again and found that it added up to 365 days. This was 6000 years ago, and before that, no one had ever known that there were 365 days in a year! The Egyptians divided this year into 12 months of 30 days each.
The Roman calendar was not the same every year, and there was a group of officials who decided how it should work. They were called “the pontifices.” Every month they would watch for the new moon. When it was seen, they would proclaim how many days were to be counted before “the nones.” This was a special day of the month. The “ides” was the day of the full moon. The first day of the month was called “the calends.”
Because each year had a different length, there was a great confusion in the calendar. In 46 B.C., Julius Caesar made changes to uniform the calendar. He ordered that the year 46 B.C., should have 445 days
But as time went on it was discovered that Easter and other holy days were not coming where they belonged in the season. Too many “extra” days had piled up. In the year 1582, the calendar was reading March 11th at the time of the Vernal equinox (which had occurred on the “March 25th in Caesar’s day). On a Solar calendar, such as an annual and predictable solar phenomena (seasons) should have been an “immovable feast”, falling on the same date every year. This should be as true as the fact that lunar phenomena (full moons) are to be “immovable feast “on a lunar calendar ( see fourteenth Nisan and the Jewish Lunar calendar). But even Caesar’s system, the calendar, and Mother Nature were not keeping in step. This was because Caesar’s allotment of 365 days per year was over six hours short. The sun was faster than Julius calculated, and by 1582 his calendar was two weeks behind.
Two astronomers, Lilius and Clavius, calculated the astronomical year to be 365, 2422 days. This inaccuracy found in the Julian calendar after its acceptance at the Council of Nicaea (A.D. 325) was corrected by Pope Gregory XIII in 1582. Pope Gregory XIII ordered that ten days should be dropped from the year 1582, thus making 1582 the year that October 15th followed October 4th immediately. And to keep the calendar accurate for all future time, he decreed that an exception to leap year would be made, with the quarter day include by adding one day every four years, “leap year”. However, the pope ordered that leap year should be skipped in the last year of every century unless that year could be divided by 400. Thus 1700, 1800 and 1900 were not leaped years, but the year 2000 was a leap year by using this simple formula, people were able to prevent the original “calendar error” from recurring (except for the one day every 3,500 years).
This system is called the Gregorian calendar and is now used all over the world for everyday purposes, though it took some time for protestant and orthodox countries to agree to the Reformed calendar. England finally adopted it in 1752 and Russia in 1918