From today's "Writer's Almanac":
Today is Leap Day, the extra day that we tack on to February every four years to keep the calendar in time with the seasons. We do this because the Earth does not orbit the sun in a nice round 365 days, but rather in 365 days, 5 hours, 48 minutes, and 45 seconds.
Ancient peoples based their calendars on many things, from the movements of the stars to the activities of plants and animals. The Greek poet Hesiod told farmers to begin the harvest when the constellation Pleiades was rising and to begin plowing when it was setting, and to sharpen their farming tools when snails began climbing up plants. Most early calendars were based on the stages of the moon, with lunar months of about 29 days each. But the problem with the lunar calendar is that it's about 11 days short of the actual year, so instead of having to add a leap day every few years, you have to add a leap month. The Egyptians were one of the first civilizations to develop a calendar with 12 months and 365 days. When Julius Caesar rose to power, the Romans were using a calendar that was so faulty they often had to add an extra 80 days to the year. In 46 B.C., after his affair with Cleopatra, Caesar chose to adopt the superior Egyptian calendar, and this became known as the Julian calendar. In the first version of the Julian calendar, February had 29 days most years and 30 days in leap years. Caesar named the month of July after himself, so when Augustus came to power, he decided he needed a month too. He named August after himself, but he had to steal a day from February in order to make August as long as July.
The Julian calendar worked well for a while, but in the 13 century, a sick old friar named Roger Bacon sent a letter to the Pope. He had calculated the actual length of the solar year as slightly less than 365.25 days, and he pointed out that the Julian calendar was adding one leap day too many for every 125 years. The result was that Christians were celebrating holy days on the wrong dates. Bacon wrote,"The calendar is intolerable to all wisdom, the horror of astronomy, and a laughing-stock from a mathematician's point of view." Bacon was eventually imprisoned for implying that the pope had been fallible, and his writings were censored. It wasn't until 1582 that Pope Gregory XIII hired a group of Jesuits to fix the calendar, and they came up with the complicated system of omitting the leap day at the beginning of each century, except for those centuries divisible by 400. When Pope Gregory made the change, the calendar was about 10 days off, so Gregory deleted 10 days from the year. People went to sleep on Thursday, Oct. 4 and woke up on Friday, Oct. 15.
At first, the Gregorian calendar was only accepted in Catholic countries, and even there people were uncomfortable about losing 10 days of their lives. It led to protests and financial uncertainty, since people weren't sure how to calculate interest or taxes or rent for a 21-day month. Protestant countries didn't adopt the new calendar until much later, and this meant that for a long time, if you crossed the border of certain European countries, you had to set your clock back or forward by at least 10 days. When Great Britain finally accepted the Gregorian calendar in 1751, 11 days had to be deleted from the year. The change led to antipapal riots, because peoplebelieved the pope had shortened their lives. Mobs gathered in the streets, chanting, "Give us back our 11 days!" When the British colonies in America made the change the following year, Ben Franklin wrote in an editorial, "Be not astonished, nor look with scorn, dear reader, at ... the loss of so much time. ... What an indulgence is here, for those who love their pillow, to lie down in peace on the second [day] of this month and not awake till the morning of the fourteenth."
The Gregorian calendar has since been accepted everywhere as the standard. It is so accurate that we will have to wait until the year4909 before our dates become out of step with the Earth's orbit by a full day.