Leap day coming on Monday. Here are a “common” year and a leap year side by side.
(Actually they are the small “block calendars” in the Astronomical Calendars for the years, with blue color denoting Moon-free nights.)
You can see that January 1 is a Thursday, then becomes in the next year a Friday; and this one-day jump is what ordinarily happens to all the dates in the year, because 365 days make 52 weeks plus one day. But after the inserted Feb. 29, all the dates take a two-day “leap”: March 1 leaps from Sunday to Tuesday, and so on. That is the presumed reason for this usage in English of the word “leap,” though it seems unproved. There are other terms for the added day: bissextile day (which takes some explaining), intercalary day.
The leap day is stuck into every 4th year (except century years not divisible by 4 – thus, 97 times in each 400 years). Why? To bring the average calendar year to 365.2425 days, closer to the solar year of 365.2422: the true seasonal year, measured by astronomy from one point in our journey around the Sun to our next arrival at that point.
If you’re born on a leap day, do you have birthdays only once every four years? We can bet that such people claim to be only 15 when they’re 60, yet have it both ways by accepting birthday gifts and greetings arpund the ends of the leapless Februaries too. The number of leap-day babies is presumably about 97/(400*365.2425) = 0.0006663942 of the world’s population; something like 4 million. Or more, if conception is above average in May.
Not rare, anyway. Rarer is for the birth on a leap day to be twins; rarer again, triplets. Or for a mother to bear a baby on a leap day, and again on another leap day. Rarer again, on a third leap day. Rarer again, for those births to be twins; rarer again, triplets. I read somewhere that Mrs. Henriksen, Norwegian, bore triplets on 1960 Feb. 29, 1964 Feb. 29, and 1968 Feb. 29. And Mrs. Louise Estes of Provo, Utah, achieved the same on the leap days of 2004, 2008, and 2012.
And beyond? What is all but impossible for physiology is easy for imagination. Quadruplets would be more appropriate, since February is the month of four weeks. Once upon a time, there was a queen who… And her children set out to find the four corners of the world. They had time to reach their goals, since they each lived to be 280.