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They Say It’s Your Birthday


Statistically, in Leap Years, about one of every 1461 Americans celebrate their quadrennial natal day on February 29. I’ve always wondered what they do to celebrate. When do they celebrate in the other three years? Are they March Fools or perhaps just twenty-eighthers? Based on a total population of 333.247 million, how many Americans do we expect to be popping their corks on February 29? To them I say, “Pop of the day to ya!”

Well, what about the rest of us? My intent is to show that people born on the first day of any month may be more rested; those born on the thirteenth, more unlucky; and those “twenty-ninthers,” perhaps a bit more industrious than the population as a whole. I hope that the details to follow help to answer the question, “can I expect my fair share of birthdays to fall on the weekends?” In truth, the answer is “not necessarily.” In fact, the 13th of the month falls more often on a Friday than any other day, and consequently the 1st more often on a Sunday. According to Guy Ottewell, producer of the Astronomical Calendar sponsored by Furman University, “Friday falls on the 13th more than any other day does ... the counts of Sundays the 13th, Mondays the 13th, etc. are: 687,685, 685, 687, 684, 688, 684” in every 400-year cycle since the adoption of the Gregorian calendar.

I’m sure you’re stunned. How could this biased system have been favored by the enlightened westerners of the so-called Enlightenment Era? It is explained by the workings of the solar system, our celestial spin zone, and the math. You would expect that over the long, long haul the calendar would “even out” and give all the days of the week an equal chance at each date in a month. After all, if a month ends on a Friday, we don’t skip to a Monday to start the next month, do we? So the uneven allocation we give the months – i.e. 30 days hath some, others 31, except February which vacillates – shouldn’t matter, and in fact it does not. If we track an 84-year period, all days fall equally on all dates. Super! At year 100 though, in comes flying one monkey wrench. We deny our 2/29 minority a birthday in three of every four century years. Yi! Kick ‘em while they’re down, why don’t ya.

Why did we adopt such a calendar? The main reason is the time it takes the Earth to complete its orbit: 365.25 days plus a “smidgen.” So a leap day every four years takes care of that 0.25, but someone figured out that the smidgen would eventually shift us away from the proper seasons. Not even 29thers want a sweltering Christmas, what with all those new electronic gadgets heating up the house. It was decided (not by me) to deny leap day on the century years except those divisible by 400; for instance, the leap day was observed in 2000, but not in 1900. This makes the calendar about as consistent with the turning old Earth as it can be, and an added “leap second” every now and then will keep it so.

What about over the long, long haul? The math tells us that we’re stuck with more Fridays the 13th than Thursdays or Saturdays. The details are as follows:

typical four-year period is 365 x 4 + 1, or 208 weeks and 5 days
typical 100-year period: 25 times the above minus one day, or 5217 weeks and 5 days
all 400-year periods are 4 times the above plus one day or exactly 20,871 weeks

That last fact clinches the deal. Since there are an exact number of weeks, every group of four centuries begins on the same day of the week and repeats the (uneven) distribution of days and dates. February 29th fell on a

Saturday in 2020, as it will again in 2420.

How do the February 29th birthdays distribute? There are a grand total of 97 in every 400 years: 15 each on Mondays and Wednesdays, 14 each on Fridays and Saturdays, and 13 each for the remaining days, including Sundays. So you see, the 29thers get fewer weekend birthdays and fewer “cazh Fridays” too. And so this month, to those folks, we all must wish a happy, happy, happy, happy birthday – plus a smidgen.

February 2008, October, 2021

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