Leap years have been part of the calendar for over two millennia, which means they (leap years and calendars both) predate English itself. So it should come as no surprise that a leap year has another name besides leap year.
The term bissextile is a hanger-on from ancient times to describe all things that have to do with that extra day added every four years. And, etymologically, the word bissextile is more clearly descriptive of what it is — at least, it would be if you were brought up in Rome 2,700 years ago.
Before the reign of Julius Caesar, Rome used a very flawed lunar calendar. In that old Roman system, the months were divided into day markers called calends (the first of the month, coinciding with the new moon), nones (the fifth or seventh day), and ides (the middle of the month). Individual days were referred to by how many days they fell before the calends, nones, and ides.
The Julian calendar, mandated by and named after Julius Caesar, was the first 365-day solar calendar. But Caesar and his astronomers recognized that a solar year wasn’t exactly 365 days long, and so every few years (and they had some calculation problems here), they added an extra day at the end of the year. This was still a time when the names of the months still made sense — the year began in March; December was, as the name implies, the tenth month; and the last two months of the year were January and February.
So under the Julian calendar, the leap day was added to the end of February. According to the old Roman system, that extra day was inserted after the sixth day before the calends of March, essentially making it the second sixth day, or bissextus, from bis-, twice or double, plus sextus, sixth.
Though both the old Roman lunar calendar and the Julian calendar have by and large been left behind, the words bissextus or bissext, the leap day, and their adjective form bissextile are still around, though not in common usage.
So, this Monday, February 29, wish your friends a Happy Bissextus!
Bonus trivia: Although that sex in the middle of bissextile is nothing untoward, some sexism has been associated with the bissextile year. An old and probably better-off-forgotten tradition says that during a leap year (and presumably only during a leap year), women are allowed to ask men to marry them. And in some corners, men were not allowed to decline such a proposal.
Leap year? Try gulp year.