There's an unusual date coming up on the calendar this week. "February 29" seems a strange thing to see, when we are used to only having 28 days in this, the shortest month of the year. And the history behind this strange, once-everyfour-years-but-not-quite date goes all the way back to (at least) Julius Caesar!
Caesar reformed the Roman calendar in 46 BCE. Before that, a Roman year was 355 days long and was divided into lunar months. After a number of years in this system, the seasons would get out of whack with the calendar date. In fact, by Caesar's time, Rome's calendar had diverged from the seasons by about three months.
Meanwhile, in Egypt, the people there had been observing a 365-day year since as early as the third century BCE and had even established the leapyear system to correct the calendar every four years. This calendar was based on the sun rather than the moon cycles.
And so Caesar decided, in consultation with the Alexandrian astronomer Sosigenes, to create his own new system, hoping to solve the problem of the season drift. First, he decreed a single, 445-day-long "Year of Confusion" in 46 BCE to correct the long years of drift all at one time. After that the new "Julian calendar" was established, with each solar year measuring 365.25 days long. Months had pretty similar lengths to ours today, and the shortest month would -- as it does this year on Saturday, Feb. 29 -- get an extra day every four years to account for the annual accumulation of a quarter of a day.
But even this system had its problems, because the ¼ of a day that leap year adds annually to the calendar is actually just a tiny bit too long. This meant that the calendar year was approximately 11 minutes shorter than the actual solar year, so the two diverged by an entire day once every 128 years. By the 16th century, this small discrepancy had caused important dates - including Christian holidays linked to the lunar calendar - to drift by about 10 days. So Pope Gregory XIII issued a Papal Bull that was meant to set things right, and, as a result, in the year 1582, Oct. 4 was followed directly by Oct. 15. Pope Gregory's plan to prevent such a maneuver from ever being necessary in the future was ingenius but confusing: Every four years would be a leap year, unless it was the beginning of a new century -- except for every 400 years, when we would keep the extra day ("the exception to the exception," as TIME magazine once called it).
And so, that's why today leap years divisible by 100, like the year 1900, are skipped unless they're also divisible by 400, like the year 2000, in which case they're observed.
So all's well that ends well, right? Well, actually one little problem remains: The current Gregorian calendar system produces an average year length of 365.2425 days, which is actually half a minute longer than the solar year. At this rate, in 3,300 years our current calendar will need to be tweaked once again! But maybe we should just leave that little problem up to future generations.