The Seasonal Almanac shares astronomical and nature events, this issue includes information on the origins of leap day and the leap day of February 29, 2000.
This February 29 is a day unique in the history of human timekeeping. It is the first time the civil calendar of the entire world puts into practice the role that says leap day shall occur in centenary years evenly divisible by 400.
Now, we all know that the origins of leap day state that leap day is supposed to be added after February 28 in every fourth year. This must be done because Earth does not take 365 1/4 days to orbit the sun, it takes closer to 365 1/4 days (we add one day in every fourth year to make up for the quarter days). But in truth, Earth doesn't take exactly 365 1/4 days to orbit the sun, either; it takes ever so slightly less time than that (365.2422). To account for the discrepancy, to 1582 Pope Gregory, relying on the advice of the astronomer Clavius, established that the extra day (leap day) would not be added in centenary years — that is, those with dates ending in "00" — except if the centenary year date was evenly divisible by 400. The dates 1700. 1800 and 1900 were not evenly divisible by 400, so people expecting a leap day in those years were disappointed. On the other hand, the dates 1600 and 2000 are evenly divisible by 400, so in the Gregorian calendar they get to keep their leap day.
But by 1600, only Catholic countries of Europe had accepted the Gregorian calendar. It took England and the American colonies over a century and a half longer to adopt it, and Greece didn't officially accept the Gregorian calendar until 1923. Therefore, the day February 29, 2000, will be a day truly unique in the history of timekeeping — at least until 2400.