A new use of an old obscure word has emerged in recent years.  With leap day approaching, we will undoubtedly hear a number of sources wishing a “Happy Bissextile Day!” for February 29th.

The problem is that the term bissextile day does not refer to February 29th, and it never did.  I assume some folks get this mistaken impression due to old dictionaries that state the definition of bissextile day as something like “the day added to leap years.”  That definition is correct, but the problem is such definitions were written back when educated people would have understood that “the day added to leap years” was February 24th, not February 29th.

“Huh?” you say.  “What do you mean, February 24th is added in leap years?  That makes no sense.”  Well, it doesn’t make sense in modern calendars, but that is in fact when the bissextile day happens in leap years.  Thus, if you want to use that fancy word, don’t use it for the 29th.  Let me explain.

What “bissextile” literally means

No, it has nothing to do with sexual orientation (or textiles), though the fact that it sounds like something more interesting than a typical calendar day probably is a reason behind the revival of this obscure word.  Some recent references seem to think it has something to do with February 29th being the 60th day of the calendar year, but that etymology is false.  The sext part of the word has nothing to do with 60.

Instead, the word comes from Latin bissextus, which comes from bis (“twice”) and sextus (“the sixth”).  What does that mean?  “The sixth” what?  And why “twice”?

We’ll get to that in a moment.  First, I must point out that this meaning is why the word can’t just be used for any inserted day in a leap year.  It’s referring to something very specific, and that thing is definitely not February 29th.  It is still accurate to say that a leap year is a “bissextile year,” because leap years contain a bissextus.  But is not correct to refer to the added date of February 29th as a “bissextile day.”

The original leap day: after February 23rd

Way back in the very early days of Rome, the calendar apparently began in March.  That’s the reason why our months September, October, November, and December come from root words for seventh, eighth, ninth, and tenth.  February was the last month of the year, having only 28 days.  The ancient Romans thought odd numbers were more lucky than even ones, so they divided up this last month into two parts (each with an odd number of days): the first 23 days were the first part of February, and the last 5 days were devoted to end-of-year festivals before the new year in March.

Originally, the months were aligned to phases of the moon, and the first day of each month was the Kalends (coming from a root meaning “called out”), the day when the new moon would be announced by the high priest of Rome.  (This is where we get our word Calendar.)  During the time of the Roman Republic, standard month lengths were adopted, so the months no longer lined up with the moon.  However, the months were shorter than now, and they added up to only 355 days.  Since the solar year is 365 or 366 days long, there weren’t enough days to line up the seasons properly.  Each year the calendar would drift a little backward in time.

The solution for the Romans was to add an extra month every few years after that first part of February, which ended on the 23rd (called the Terminalia).  This was known as an intercalation, i.e., inserting an extra Kalends into the year.

This practice of adding an extra month was often subject to imprecision, manipulation for the sake of luck (the Romans often viewed intercalation as unlucky), and even political whim.  (Important elected officials often served for a year, so a priest could add the extra month in a year when allies were in office or leave it out in years when enemies were serving.)  Eventually, these disputes were resolved by Julius Caesar, who instituted what we now call the Julian Calendar.  Before he did this, the irregular intercalations had the calendar so messed up that Caesar had to lengthen the year 46 BCE to 445 days, just to line up the seasons again.  (This year is sometimes known as the “Year of Confusion.”)

Caesar’s calendar reform added days to various months, resulting in a standard year length of 365 days.  He, along with astronomers of the time, determined that an addition of an extra day every four years would keep the seasons approximately aligned correctly.

When did he add that extra day?  Well, the same place that intercalations had always happened, of course: on the day after February 23rd.

Why the Roman leap day is “twice” the “sixth”

Now we know why technically the extra day occurs on February 24th.  But why is it known as bissextile, i.e., “twice” the “sixth”?

This name goes back to the old Roman calendar again.  Recall that the Kalends was the day that the new month was announced with the new moon.  Since the Romans could see the moon waning, they could predict when the next month would begin following the moon cycle, beginning around the time of the full moon.  Thus, days from the middle of the Roman month onward were actually counted backward from the next new moon (Kalends).  The Romans rarely numbered the days of the month in ordered dates as we do today.  Instead, the last day of the previous month would be called “the day before the Kalends.”  The day before that would be called “the third day before the Kalends.”  (Today, we’d called it the “second day before,” but the Romans always counted inclusively, starting with the day they were counting from.  So, the Kalends itself was the first day, the day before the Kalends was the second day, and the day before that was the third day.)

February 24th was thus the “sixth” (sextus) day counting backwards starting on the Kalends of March.  In leap years, this date would become a problem.  You’d start on February 23rd, which was in Roman terms the “seventh day before the Kalends” in normal years, and then you’d insert the extra day (February 24th) as that was the typical place to do so, and then you’d move to the next day (what we’d called February 25th in a leap year), which was the regular “sixth day before the Kalends.”

But how to number February 24th?  The numbering skipped from the “seventh day before the Kalends” to the “sixth day before the Kalends,” but you sometimes would have to call that day in the middle something.

And the Romans decided they would just count the “sixth day before the Kalends” twice.  Hence, the bissextus, or “twice the sixth.”  February 24th would be the first sixth day” and February 25th would be the second “sixth day.”  (Yes, it’s confusing, but that’s because we’re not used to counting in this fashion.)

But we use the Gregorian Calendar now and count forward, so why care about the Romans?

Well, first it’s important to note that the Gregorian Calendar (introduced by Pope Gregory in 1582) still referred to the leap day or bissextile day in its traditional place as the day following February 23rd.  Traditional secular calendars and church calendars followed that practice, with the Catholic Church making appropriate liturgical adjustments: saints days celebrated on February 24th through 28th always moved forward by one day in leap years.  (This practice continues in Catholic churches today, though the only major saint day celebrated in those dates was moved in the latest version of the calendar in 1970.  Most Catholics would only notice this change in local saints celebrated on the 24th through the 28th.)

The Roman dating practice of counting back from the 1st of the month was still common in medieval and renaissance Europe.  It wasn’t until the 1600s or 1700s that it was completely dropped in most places in favor of our modern system of counting days consecutively forward from the beginning of the month.  The placement of the bissextile day was never officially changed from February 24th, but due to our new system of counting, the “leap day” was increasingly seen as occurring on February 29th, a number which did not occur outside of leap years.

In any case, there’s really no reason to care about any of this with our modern calendar, unless you are still trying to understand the subtleties of saints days in liturgical calendars or something.  The only reason to know about it today is because some people have bizarrely revived the term “bissextile day” and don’t realize what it means or that it really refers to February 24th.

Celebrate on “leap day” if you wish, just don’t call it “bissextile”

Nowadays, February 29th is almost universally regarded as the “extra day” added in a leap year.  It has a perfectly reasonable and straightforward term: Leap Day.  Wish people a “Happy Leap Day!” if you’d like on February 29th.

And if you want to sound erudite, you can talk about the year as a bissextile year, since it still does contain that extra day as designed in both the Julian and Gregorian Calendars.

But please don’t refer to February 29th as a bissextile day or wish anyone a “Happy Bissextile Day!” on the 29th.  First, it sounds pretentious.  Second, it’s wrong.  Third, it makes no sense.  The word doesn’t just refer to any leap day.  Its name describes exactly what it is: the double “sixth” day before the beginning of March, i.e., February 24th (and its cousin February 25th).  If you truly want to have a bissextile celebration, why not have a two-day party spanning both the 24th and 25th?  That would truly be giving that “twice sixth” confluence of two days its due.

In any case, have a happy Leap Day on February 29th!