Felix Bissextilis!

It is Leap Year Day.  In Latin this is bissextilis.

For this intercalary day in the Roman Martyology we find this:


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When there isn’t a 29 February, the saints are observed on 28 February.

In 46 BC, on the advice of the Alexandrian astronomer Sosigenes, G. Iulius Caesar created a calendar system that added one day every four years to make up for the fact that the Earth’s year is slightly more than 365 days. The Earth circle the Sun in slightly more time than it takes for the Earth to rotate 365 times (365.24219). Calendar years with 365 drift from the actual year by about 1 day every 4 years.  After a while the month named after Caesar, July occur during the winter (in the Northern hemisphere).

Caesar’s Julian Calendar was maintained until 1582 when Pope Gregory XIII determined that in his Gregorian Calendar leap days would not occur in years ending in 00, unless the year is divisible by 400.

The feast of St. Matthias in the old calendar fell on the 24th of February in common years, but in leap years, because of bis sextilis, it was celebrated on the 25th.

Let’s just say that it’s complicated.

First, when the Roman Senate renamed the month Sextilis to honor Augustus, they borrowed time from February to make August longer than July (after Julius, formerly known as the fifth month, Quintilis).  So, February wound up being shortened.

Sextilis the day was six days before the kalends (1st) of March (inclusive) = 24 February. The intercalary month that the Romans used to try to keep the civil calendar in sync with the solar calendar was inserted behind one of the festival of Terminalia on the 23rd. In the reform of the calendar, one day was inserted behind the 23rd of February every three or four years. It was the “double sixth day” or bis sextilis. Bis sextilis and sextilis (the 24th and 25th to us) were considered as one day long day.

The feast of St. Matthias in the old calendar fell on the 24th of February in common years, but in leap years, because of bis sextilis, it was celebrated on the 25th.

Whereas in most years the calendar is advanced one day at a time, in a leap year there is a week in which a day is advanced two days.  In 2012 Christmas (25 December) was a Tuesday.  In 2013 a Wednesday.  In 2014 a Thursday.  In 2015 a Friday.  In 2016 Christmas falls on Sunday, not Saturday.  You can see the “leap” in the calendar.

Anyway… it’s complicated.

Felix Bissextilis!


I received a good explanation in an email from the British Library:

In 1582, calendrical reform came from Rome again, this time, from Pope Gregory XIII (1502-1585). Gregory realised that because a whole day was added to every fourth year, when in fact it should be a bit less than a day to be accurate, the Julian calendar was 11 days ahead: 15th October in Gregory’s time was, astronomically, 4th October. In order to cut out this accumulated surplus, he issued a Papal directive stating that 4th October in 1582 will be followed by 15th October and the first year of each century will not be a leap year any more, except if it is divisible by 400. So what about the leaping saint? Well, the medieval solution for the leap-year problem was generous. By doubling 24th February the following saints’ feast days could all keep their original date and – because there were two 24ths in the month – February remained 28 days long. In this way, no saint suffered the ignominy of having their feast day celebrated only one year in every four. Instead, there was a gain: in the leap year Saint Matthias was celebrated twice – on the 24th(a) and 24th(b) alike.

Yet curiously, in this overhaul the repeated 24th remained in place. It was only over time that the medieval system of two 24ths was phased out and replaced by a 29th day of the month, but the tradition of having an extra 24th with its leaping saint, the Apostle Matthias, is still preserved in the Catholic liturgy.


A reader wrote with an interesting factoid.

Apparently Ash Wednesday as not yet fallen on a 29 February and it won’t until 2096.

Please share!

About Fr. John Zuhlsdorf

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12 Responses to Felix Bissextilis!

  1. Gregg the Obscure says:

    In the archdiocesan calendar here is a note that today is the anniversary of the 1976 ordination as a bishop for Francis Cardinal Stafford (he served here in the 80s and 90s before being named a cardinal). So he’s been a bishop for 40 years, but it’s strictly speaking just the tenth anniversary of his ordination as such.

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  3. Dr. Edward Peters says:

    Bissextilis. Okay, there’s my word for today. Thx.

  4. Edward Mulholland says:

    INcidentally, the Gregorian calendar reform also explains why St. Teresa of Avila’s feast day is not October 5th. She fell ill and was waning, but still alive, the night of October 4th. She died hours later. But since it was 1582, October 5th through 14th never existed… so her death day, and feast day, is October 15th. (That was good news for St. Placidus and companions who were not upstaged by a more popular saint on October 5th.)

  5. Grateful to be Catholic says:

    The email from the British Library says “the first year of each century will not be a leap year any more, except if it is divisible by 400.” The first year of every century ends in 01 and is never a leap year and is never divisible by 400. The way it was stated earlier by Fr. Z is correct: years ending in 00 are not leap years unless they are divisible by 400. I realize we have not had to have this discussion about when a century begins since 2000 ( a leap year) and will probably not need it again until 2100 (not a leap year), but it is good to keep in practice.

  6. Blaise says:

    I believe the change from Julian to Gregorian calendar is also why here in the UK the tax year (for individuals) begins on 6th April. It had previously been 25th March,the Annunciation, as the quarter day but with the loss of 14 days the crown not wanting to lose out, moved this to 6th April. Of course this could be a complete misunderstanding on my part.

    I love the thought of a time when the keye feasts of the Church’s year marked life generally. Although I am not sure that making them the focus for paying your rent or other quarterly financial events is exactly celebrating: “happy Christmas,now where is your rent?” is not my idea of celebrating.

  7. leftycbd says:

    @Blaise: The British Empire celebrated New Years’ Day on the feast of the Annunciation, March 25th, until the time they adopted the Gregorian Calendar in 1752. By this time, the calendar was behind by 11 days, and thus 11 days were dropped instead of 10 as was done in 1582 by Catholic countries. (For you Linux/Unix geeks, go to a terminal and type: “cal 9 1752”, and “ncal -p”).

    The beginning of the tax year was March 25th, as it was the first day of the year. In order to not lose any revenue, the Treasury moved the start of the tax year forward by 11 days, which was April 5th, so that the tax year of 1752 still had 365 days, even though the calendar year had 354 days that year. Thus the calendar year now started on January 1st, but for tax purposes it started on
    March 25th + 11 days = April 5th.

    Then 48 years later, it was 1800. which in the old Julian calendar had 366 days, but in the new Gregorian calendar, it only had 365 days. So as not to lose that day of revenue that they would have had, they moved the start of the tax year to April 6th, so that the tax year of 1800 had 366 days!

    That is why today the UK’s tax year starts on April 6th and not January 1st.

  8. kurtmasur says:

    So, ecclesiastically speaking, the leap day (or “bissextilis) was not today 29.02, but last Thursday and Friday together being technically the same day. The folks at the New Liturgical Movement dedicated that day’s blog entry (for Friday) about this little fact. They also pointed out an interesting fact that just about all the romance languages to this very day still conserve the latin “bissextilis” (or rather, a derived variation of it) when referring to “leap year”, in spite of observing the 29.02 (and not an extra 24.02) as the true “leap day”.

  9. leftycbd says:

    One more thing on leap years. The Gregorian calendar is off by 3.1203708 days every 400 years. We take care of the 3 days by dropping 3 leap years every 400 years.

    That still leaves us with 0.1203708 years to deal with every 400 years. Multiplying by 10, we will be off by 1.203708 days in 4000 years. 4000/1.203708 ~ 3323 years until exactly 1 day of drift.

    Starting from 1582, we will have to drop an extra leap year by the year 4905. (1582 + 3323).

  10. mpolo says:

    I noticed a weirdness for the first time this year. In Germany, St. Matthias is still celebrated on the 24th (due to his grave being in Trier), but in the Ordinary Form (in German), the date of the 24th is maintained in leap years.

    However, the General Instruction for the Liturgy of the Hours in Latin seems to indicate that saints from the 24th–28th should be celebrated on the 25th–29th in leap years. (Just that there are no saints in the General Roman Calendar for those days.)

    I ended up celebrating St. Matthias on the 24th with an ordinary form Mass and Liturgy of the Hours, but had doubts about the “correctness”. However, I knew that I had a vernacular Mass to celebrate on the 25th, and I would have missed the saint entirely if I hadn’t done this.

  11. padresgo says:

    Funny you should post this term today (which I had never before heard of). Just last night (Feb. 29) I had a meeting with my parish Guadalupe Committee to plan for this years’s events. I asked them if there was a word in Spanish for what some English-speakers now seem to refer to as “Leap-Day”. They told me in Mexico they call it el “Biciesto”. I had no idea what may have been the origin of the word until I read this post today. Thanks! I do find a great deal of classical terminology has been preserved among Mexicans in way I don’t see among English-speakers.

  12. Venerator Sti Lot says:

    Carrying on from Blaise and leftycbd, the other two quarter days were St. John (24 June) and Michaelmas (29 September). But, as Blaise says, so well-known were saints’ days (or, at least some of them), that charters would use other pairs of saints if a definite rent were due at some times other than the quarter days – for example, twice a year at the Conversion of St. Paul (25 January) and at St. James (25 July) – easy, because in six months of the year a great feast fell on the Eighth of the Kalends, and in two others on the Seventh or Ninth. In England, though, there was one such day commonly used that was connected with a moveable feast – the Tuesday after the Octave of Easter, called ‘Hock’ or ‘Hoke’ Day (‘Hocktide’ being that day and the preceding Monday): treated as a convenient time for spring payments when the autumn payments were at Michaelmas. (The Exchequer opened its two annual sessions on Hock Monday and the morrow of Michaelmas.)

    It was another Pope Gregory – the Great – who introduced the ‘modern’ way of counting the days straight on from the first to the last day of the month, in the west: in his chancery, but his successors went back to kalends and ides, etc., though, curiously, it caught on among the Lombards and Franks – except that Charlemagne then influentially favored the classical system again.

    And then there’s the ‘consuetudo Bononiensis’, which somehow got started in Bologna, counting up halfway through each month from the ‘intrante mense’ and then down backwards to (or, up backwards from, depending on how you look at it) the ‘exeunte’ or ‘stante mense’. (But I don’t know what they did, or didn’t do, about leap year.)