Leap day, also called an intercalary day, is a day that is periodically added to a calendar year to keep it in alignment with the solar year. In the Gregorian calendar followed throughout the world, a leap day is added almost every year that is evenly divisible by four. The extra day falls in February.
The solar year is the time between two passages of the sun through the March equinox. In the Northern Hemisphere, this equinox is called the vernal equinox. The solar year is 365 days, 5 hours, 48 minutes, and 46 seconds long. The solar year is the basis of our calendar year.
The calendar year is only 365 days long, however, shorter than the solar year. As a result, without any corrections, calendar dates would drift forward relative to the solar year. In roughly 750 years, the months in which the seasons fell would be completely reversed, such that winter would occur from June through August in the Northern Hemisphere.
An extra day is added every four years to correct the difference between the calendar year and the solar year. This fourth year is called a leap year , and the extra day is called an intercalary day or leap day.
Adding a leap day every fourth year makes the average calendar year 11 minutes and 14 seconds too long. So, the leap day is not added in the century years, except in those divisible by 400. The years 1700, 1800, and 1900, for example, had only 365 days. The year 2000, on the other hand, had 366 days.
Why it is called a leap day.
On a 365-day calendar, fixed-date holidays—holidays that always occur on the same date—progress through the days of the week from year to year. For example, if Christmas , which is fixed to December 25, occurs on a Tuesday one year, it will occur on a Wednesday the next year. If that next year has an intercalary day, however, Christmas will “leap” Wednesday and fall on a Thursday instead. A year with an intercalary day is therefore called a leap year, and the intercalary day itself is called a leap day.
History.
Ancient civilizations developed calendars by studying the movements of the sun, moon, and stars. But these calendars rarely maintained their alignment with the solar year. Some civilizations added intercalary days to keep their calendars from drifting in relation to the solar year.
The modern leap day has its origins in calendar reforms enacted by the Roman ruler Julius Caesar . Errors in Rome’s earlier calendar, and the occasional failure by Roman religious officials to add extra days at the proper times, had pushed the calendar about three months ahead of the seasons. Caesar added days in the year 46 B.C. to bring the calendar back into alignment with the solar year. He also created reforms intended to maintain the calendar’s alignment. The Julian calendar divided the year into 12 months, of which seven had 31 days, four had 30 days, and one—February—had 28 days. Every four years, February was to have 29 days.
The Julian calendar represented a major improvement over the Roman Republican calendar previously in use. But Julius Caesar was assassinated shortly after instituting it, and later rulers failed to implement it properly. Furthermore, the average Julian calendar year remained 11 minutes and 14 seconds too long.
By A.D. 1580, the difference between the Julian calendar and the solar year had accumulated to 10 days. In 1582, the Roman Catholic Pope Gregory XIII dropped 10 days from October to make the calendar year correspond more closely to the solar year. He reaffirmed that each fourth year would be a leap year, when February would have an extra day. To prevent later drift, Gregory decreed that century years not evenly divisible by 400 were not to include leap days.
Placement of the leap day.
Today, most people regard the 29th day of February as the leap day, because that date only occurs on leap years. For hundreds of years, however, most followers of the Julian and Gregorian calendars would have regarded the 24th day of February as the leap day.
The Romans did not number the days of the month consecutively, as we do today. Instead, Romans described a day by the number of days from the next of three special days of each month: (1) the kalends, which was the first day of the month; (2) the nones, either the fifth or seventh day of the month; and (3) the ides , the 13th or 15th day of the month. For example, March 12 instead would have been designated the 4th day before the ides of March. The ides of March was the 15th day of that month.
Prior to the reforms of Julius Caesar, Roman officials had occasionally inserted an intercalary month into the calendar. This month began after the sixth day before the kalends of March, the 23rd day of February. Caesar preserved this placement for the intercalary day in the Julian calendar.
Many regions that used the Julian calendar did not share the cultural and religious practices that led the Romans to use such a complex month-counting system. Some of these regions began using the simpler style of counting the days forward from the first day of the month in the 400’s. The forward-counting style gradually replaced the Roman style over more than 1,000 years. As the world switched to the forward-counting style, people began to consider the 29th day of February as the leap day, rather than the 24th.
