28 Days

Why does February have 28 days when all the other months have 30 or 31? The question may not seem to have much to do with word origins, but when you start digging into the answer you uncover a trove of word origins relating the calendar.

The first is the origin of the word calendar itself. In the Roman system of reckoning dates the kalendae, or kalends, was the first day of the month. (The singular form kalend or calend is sometimes found English, but in Latin the word is always plural.) Kalendae literally means accounts, and debts were due on the first day of the month, hence the name.

The Roman calendar had two other named days each month. The nonae, or nones, was the seventh day of the month in March, May, July, and October and the fifth day in other months. The nones, meaning ninth, is so called because it falls nine days before the ides (when you count inclusively). So the ides would fall on the 13th or 15th, depending on the month. The ides was originally the date of the full moon, making the nones the day of the half moon, but at some point these dates were fixed. The etymology of ides is uncertain, but is most likely of Etruscan origin and unknown meaning. The Ides of March is famous because that is the day on which Julius Caesar was assassinated and because of the famous warning, Beware the Ides of March, that appears in the Shakespeare play.

Our modern calendar is based on the Roman one, which originally had ten months of 30 or 31 days each, with a 61 day period between December and March that fell outside the calendar. This gap was presumably because the calendar was chiefly used to regulate planting and harvesting and this period was unimportant to farmers. The later addition of two more months explains why the numerical Latin roots of the months’ names are two off from their position on the calendar. October was originally the eighth month, September the ninth, etc. The original Roman months were:

  • Martius (31 days), named for the god Mars
  • Aprilis (30 days), after Apru, the Etruscan name for Venus
  • Maius (31 days), after the goddess Maia
  • Junius (30 days), after the goddess Juno
  • Quintilis (31 days), the fifth month
  • Sextilis (30 days), the sixth month
  • September (30 days), the seventh month
  • October (30 days), the eighth month
  • November (30 days), the ninth month
  • December (30 days), the tenth month

The months of January (Ianuarius), named for the god Janus, and February (Februarius), from a Sabine word meaning purification because of religious rituals conducted during that month, were added during the reign of Numa Pompilius, the second king of Rome (c.715-673 B.C.), bringing the entire year within the calendar. But in Roman reckoning, these were the last two months of the year, not the first. The Roman new year began in March.

Under Pompilius’s calendar, the months that previously had 30 days were reduced to 29. Ianuarius also had 29 days and Februarius had 28. In alternate years an extra month, Mercedonius, was added at the end of February to align the calendar with the solar year. In such years February was cut short to 23 or 24 days, to create an average year of approximately 365 days.

Unsurprisingly, this complex calendar was difficult to maintain. Finally in 45 B.C. Julius Caesar implemented a major calendar reform. He, or more accurately his astronomers, added ten days to the calendar year, bringing it into line with the solar year and eliminating the need for the biennial intercalary month of Mercedonius. He set the length of the months to those we’re familiar with today and instituted the practice of the modern leap day, an extra 29th day of February every fourth year. During the reign of Augustus, the month of Quintilius, the month of Julius Caesar’s birth, was renamed Iulius or as we call it today, July, and Sextilis was named Augustus after the current Emperor.

There is a legend that July was given 31 days, with the extra day taken away from February, so that no month would be shorter than the one named after the great Julius. And not to be outdone by his adoptive father, Augustus made his month 31 days long as well, again taking the day away from February. It sounds good, but it’s not true. July and August both had 31 days and February 28 before the former two were renamed after the Roman leaders.

This Julian calendar was a significant improvement, but it was not perfect. The most significant problem is that the average Julian year of 365.25 days is 10 minutes, 48 seconds too long. The earth actually takes 365 days, 5 hours, 49 minutes, and 12 seconds (or 365.2425 days) to orbit the sun. This may not seem like much, but over the centuries the error accumulated.

As a result, the vernal equinox slowly drifted to an earlier date each year. This caused a problem for the Christian church, which calculated the date of Easter based on the equinox. On the command of Pope Gregory XIII, the calendar was revised in 1582. Days were skipped, returning the equinox to its traditional date. And to keep the calendar on track in the future, century years skipped the leap day on 29 February unless the year is divisible by 400 (1600 and 2000 had leap days, but 1700, 1800, and 1900 did not). This new Gregorian calendar also moved the new year to the beginning of January.

The Gregorian calendar was not implemented all at once. Catholic countries tended to adopt it early, Protestant ones later. Great Britain and its American colonies did not adopt it until 1752. Russia did not adopt the Gregorian calendar until after its revolution of 1918. If you do genealogical or historical research, you will often see dates prior to 1752 marked as either O.S., old style, or N.S., new style. O.S. dates are those of the original Julian calendar. Those marked N.S. have been adjusted to match our modern Gregorian calendar. George Washington, for example, was born on 11 February 1731 O.S. according to the Julian calendar which was in force in Virginia at the time, but today that date corresponds to 22 February 1732 N.S. The shift of 11 days is due to the skipping of days in order to return the equinox to its traditional date and the shift in the year is due to the move of New Year’s to January.

The Gregorian reforms brought the calendar very close to the actual solar year, but it is not quite accurate. Modern astronomers periodically add leap seconds to the clock and calendar to keep them in line with the earth’s rotation around its axis (which is not precisely 24 hours) and its orbit around the sun. Most of us don’t even notice when this is done.

See what you get by asking a silly little question like why does February have 28 days?

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