The year is the time it takes the earth to make one orbit around the sun. It is, of course, 365 ¼ days long.
This ¼ of a day creates a problem with the calendar since the calendar must have a whole number of days. The Julian calendar was one of the first to adequately address this problem.
The Julian calendar was instituted by Julius Caesar in 45 BC. In this calendar each year is considered 365 days long. This means a ¼ of a day is left out each year. After four years, a whole day has been left out, so this extra day is put back in each fourth year. This is called leap year and is 366 days long. Now the year will average 365 ¼ days.
Before this system could be put into effect, an adjustment had to be made. The current calendar was 80 days out of step. So Julius Caesar decreed that 45 BC would be 445 days long. The new system began the next year, 44 BC.
To determine if a year is leap year, you simply divide the year number by four. If it can be divided with no remainder, then it is a leap year.
This worked fine, for a while. But the year is actually 11 minutes short of 365 ¼ days. That doesn't sound like much, but over hundreds of years it adds up. By the 1500s the calendar was about 11 days out of step with the actual seasons. So another adjustment was needed and this came in the form of the Gregorian calendar.
This new calendar, instituted by Pope Gregory in 1585, worked just like the Julian calendar except for those years that end in 00. According to the Julian calendar these would be leap years since they are evenly divisible by four. In the Gregorian calendar, however, these are leap years only if they are divisible by 400. Thus, 1600 was a leap year, but 1700, 1800, and 1900 were not. The year 2000 will be a leap year.
This time another adjustment was needed. Eleven days were dropped from the year to get the calendar back in step with the seasons.
The predominantly Catholic countries adopted the Gregorian calendar immediately. Protestant countries, like England, did not.
In 1752 England and her possessions (including the USA) and most other countries finally adopted the new calendar. Russia and China, however, did not change until the early part of the twentieth century.
last update Dec 31, 1997