The current year, 1996, is referred to as a "leap year" because we have inserted a "leap day" to make the length 366 days rather than the usual 365 days. The official name of the "leap day" is an intercalary day (with the accent on the second syllable). Intercalary is the adjective form of the verb to intercalate, which means to insert. Once this day is inserted, or intercalated, it becomes an embolistic day. Still another name for this extra day every fourth year is the bissextile day, meaning a double sixth day.
This last name is derived from the location of the intercalary day every fourth year in the Julian calendar that was put into use by Julius Caesar in 45 B.C. He realized that a tropical year, the interval of time between successive beginnings of spring, is about 365 1/4 days. Since agriculture was the main occupation of most people in the world until 200 years ago, adjusting the calendar year to fit the seasons seems the most reasonable way to form a calendar. The best one can do is to make the average length of a year 365 1/4 days by having 366 days in every fourth year. Caesar's intercalary day was inserted on what is now the day before February 24, thus making a second, sixth day (bissextile) counting back from March 1 -- the beginning of the Roman year. (The Roman days of the month were not numbered as in our modern calendar.) In 533 A.D., when the monk Dionysius Exiguss determined the year of Christ's birth and began the numbering system for years A.D., it worked out conveniently that years evenly divisible by four were leap years.
Unfortunately, 365 1/4 days is not the exact length of a tropical year. The time between successive instants of the beginning of spring is 365 days, 5 hours, 48 minutes, and 46.02 seconds. While this makes the Julian year only 11 minutes and 13.92 seconds longer than a tropical year, it amounts to spring beginning one day earlier after about 128 years. The seasons were slowly moving backward through the year under the Julian calendar.
In 1582, Pope Gregory XIII was informed by his astronomer, Christopher Clavius, that the first day of spring had fallen on March 10 of that year. Since the first day of spring had fallen on March 21 in 325 A.D. when the Council of Nicaea had established the dates for the holidays in the Christian calendar, Gregory felt it was important to put the seasons back in the same places in the calendar. He therefore declared that the next day after October 4, 1582, was October 15, 1582. He also adopted a revision in the calendar which resulted in the average length of a year being closer to the length of a tropical year. This calendar, which we now use, is called the Gregorian calendar. It differs from the Julian calendar by letting century years, such as 1600, 1700, 1800, etc., be leap years only if they are evenly divisible by 400. Thus, 1600 was a leap year, and 2000 will be, too, but 1700, 1800, and 1900 were not leap years.
The average length of our Gregorian year is 365 days, 5 hours, 49 minutes, and 12 seconds, still about 26 seconds longer than a tropical year. However, more than 3000 years must now pass for the seasons to move back by one day. A suggested modification to the Gregorian calendar is to eliminate as leap years those years which are evenly divisible by 4000. This would result in the length of a year, averaged over 4000 years, being only 4 seconds longer than a tropical year. Fortunately, we still have a couple thousand years before we need to give serious thought to this modification.