Today is Bissextile Day! What is that?
Have you ever wondered why we have leap years, and why, on the traditional Catholic calendar, the feasts of late February shift a day in leap years?
Read on.
Of course, today we use the Gregorian calendar, promulgated by Pope Gregory XIII in 1582. It replaced the earlier Julian calendar that dated to the Roman era, but proved less than accurate, and the error between date and solar events such as the equinoxes, necessitated a new calendar. The root of the Julian Calendar error is this: it presumed that the year was 365.25 days long, meaning that a leap year every four years would account for the decimal places and keep the calendar year in sync with the actual solar year. As it happens, the year is more precisely 365.2422 days long, meaning that the seasons would slowly drift away from their calendar dates with the Julian Calendar -- for instance, by 1582, the Vernal Equinox was occurring on 11 March, rather than 21 March as is traditionally assumed. So, the new Gregorian Calendar restored the Equinox to its traditional date by dropping 10 days that October of 1582. It would try to remain accurate by modifying the reckoning of leap years: it would have a leap year every year divisible by 4, except those divisible by 100 (most years such as 1700AD are not leap years), but if divisible by 400, remaining a leap year (so 1600AD was a leap year). This is still a hair off, and some have suggested that we waive the leap year in 4000AD to fix the problem.
At this point, the leap day, when added, is 29 February. Formerly, however, the extra day was not inserted in that last place of the month of February, but on the "sixth day before the Kalends of March." The Roman manner of inserting a leap day was this -- simply double that sixth day before the Kalends of March. You would have the sixth day twice in a row -- hence the term Bissextile, giving you ante diem bis sextum Kalendas Martias; the second sixth day before the Kalends of March. Of course, the Romans reckoned their dates by counting down to the Kalends (First of the Month), Nones (5th or 7th of the Month), and Ides (13th or 15th of the Month). The countdown was inclusive, so the third day before the Kalends of January, would be 30 December, by our sequential method. This site has a wonderful chart showing the equivalent dates between our sequential reckoning and the Roman method: Roman Calendar: Conversion to our Calendar
The bissextile day, or leap day, then, was, considered from this perspective, that sixth day before the first of March, inclusive, or as we call it in normal years, 24 February. When that day was "doubled" you had two legal "sixth days." Sequentially, however, that meant that the 25 day of the month was now the actual sixth day before the Kalends of March, and the 29th of February the day before the Kalends.
All of this has significance for the traditional Catholic feasts that fall on 24 February or later in this month. Since, in leap years, the 24th was "doubled," in order for a feast day like that of St. Matthias the Apostle to remain on "the sixth day before the Kalends of March" it would migrate to 25 February that year. The same would go for the other feasts at the end of the month -- traditionally their place was reckoned based on their number of days from the Kalends. So, St. Gabriel of Our Lady of Sorrows typically has his traditional feast on 27 February, which is the third day before the Kalends of March, but in order to be on the third day before the Kalends of March in a leap year, it must shift to 28 February.
With the reform of the calendar in 1970, all feasts were reckoned by the sequential numbering, and the issue disappeared, much like the Roman technique of counting down to a date rather than sequentially counting up.
The great Fr. Z. has posted on this very subject, noting pertinent details that I did not go into: Fr. Z: Felix Bissextilis!
Tempus fugit!
Live well!
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