The leap year, in which an extra day is intercalated every four years into the 365-day calendar, is a device that keeps the calendar closer to the actual time the Earth takes to complete one revolution around the Sun: slightly more than 365 days.

When Julius Caesar overhauled the calendar of the Roman Republic in 46 B.C., he introduced the extra day in February every four years. Unknown to the ancients, however, a year under the new system, which became known as the Julian calendar, remained 11 minutes, 14 seconds longer than the solar year. The discrepancy, at first insignificant, grew over the centuries. By 1580 spring arrived on March 11, fully 10 days too early, disrupting farmers' schedules and the dates of Christian holy days such as Ash Wednesday and Easter.

Pope Gregory XIII forced the calendar back in line with the solar year by voiding 10 days from the 1580 calendar and - more importantly - by refining the definition of a leap year. Henceforth, years at the turn of a century would not gain a day in February unless evenly divisible by 400; while 1600 and 2000 were leap years, 1700, 1800 and 1900 were not. The reform produced the Gregorian calendar, which shaved the difference between the solar and calendar year to about 26 seconds.

Despite the virtues of the new calendar, sectarian bitterness among Catholic, Protestant and Eastern Orthodox nations prevented many countries from accepting any papal innovation, however useful.

Great Britain and its American colonies did not adopt the change until 1752; the Feb. 22 birthday of George Washington, later to become the first U.S. president, was Feb. 11 under the Julian system. The last holdout in the Western world was Russia, which observed the old calendar until after the Communist coup in November 1917.

Source: Irene Cockrott, World Book Online Reference Center. http://www.worldbookonline.com/wb/Article?id=ar087720.