Today is the day that, according to our modern (Gregorian) calendar, we insert an extra day in every year evenly divisible by four, except for centennial years unless divisible by 400. We do this because the earth’s revolution around the sun and its rotation on its axis are independent (the solar system didn’t require that they be linked so that when we’ve reached the end of x number of axial rotations we would simultaneously have completed one orbital revolution. That would be coincidence indeed.); in fact, they’re slightly out of sync.
That is, when we’ve completed one orbit around the sun, we’ve completed roughly 365.25 rotations on our axis, thus necessitating the insertion of an extra day every fourth year to keep our calendar aligned with the seasons.
That an intercalary day is required is an accident of nature.
That our extra day (an intercalated day) takes place at the end of February is a historical accident occasioned by the fact that February is already a mutilated month, having lost several of its days to the required prestige of Julius and Augustus Caesar, and it just makes sense, if you’re adding a day to a year, you add it to the shortest month you’ve got (unless you just call it “extra day” and stick it at the end, or close to one of the solstices or equinoxes, or some other radical idea).
That it’s called a leap year is a result of the 1604 Anglican prayer book , which supposedly says that “On every fourth year, the Sunday Letter leapeth.” I’ve yet to find this phrase in that book, though, finding it instead in numerous 19th-century sources, like this paragraph from Whatly:
When the years of our Lord (i. e. when the number of years from the birth of Christ) may he divided into four even parts, which is every fourth year, then the Sunday letter leapeth; and that year the Psalms and Lessons, which serve for the twenty-third day of February, shall be read again the day following, except it be Sunday, which hath proper Lessons from the Old Testament appointed in the table to serve to that purpose.
You’ll notice that the “leap day” in the Anglican book is scheduled for the 24th of February, rather than the 28th. This matched Roman Catholic practice until 1662, when the Anglicans decided to move the intercalary day to the day after the 28th (ie the 29th) instead of inserting it between two days of the same month. It doesn’t affect much, except the timing of certain feast days (like that of Saint Matthias) in late February that are celebrated on different days between the two churches in leap years.
Whatever you celebrate, and however you do it, enjoy this extra day!
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