GeekDad Puzzle of the Week Solution – Easter Anniversary Dates

Reading Time: 2 minutes

This past week’s puzzle:

wedding_main
This past Sunday was a very fun day in our house. Not only was it Easter Sunday, with an egg hunt and a great service at our local house of worship, but it was also our 12th wedding anniversary. Yes, that’s right, my wedding anniversary date is 4/20. Please note that the only reason that particular date was chosen was that it was the date when the adorable bed & breakfast at which we were wed was available.

In any case, our first wedding anniversary back on April 20, 2003, also fell on an Easter Sunday — much to our chagrin, as we wanted to “celebrate big” but couldn’t, as most restaurants were closed. More recently, in 2007 and again in 2012, there was a Friday the 13th in April — on the Friday immediately before our wedding anniversary.

Thinking about these date anomalies around our wedding anniversary led me to this week’s GeekDad Puzzle of the Week: Since the Gregorian Calendar was established in October 1582, how many times has my wedding anniversary date of April 20th fallen on either Easter Sunday or in an April that also contained a Friday the 13th?

Most solutions to this puzzle broke the problem into two parts: the 20 April Easters and the Friday the 13th in April.

As Easter Sunday is more of an “agreed upon” date than a “calculated” date, several different sources online are available for a canonical list of Easter dates from 1583 to present. Parsing through a few of those gets some 12 years with Easter Sunday dates of 20 April: 1631, 1642, 1710, 1783, 1794, 1851, 1862, 1919, 1924, 1930, 2003, and 2014.

Fridays on the 13th of the month, while calculable, are also searchable. There are several sites that list dates of the 13th that fall on Fridays (much to the delight of friggatriskaidekaphobics), and some even allow you to filter by months. Using a few of these (to gain concurrence), it is straightforward to find the 63 Friday, April 13ths between 1583 and present: 1584, 1590, 1601, 1607, 1612, 1618, 1629, 1635, 1640, 1646, 1657, 1663, 1668, 1674, 1685, 1691, 1696, 1703, 1708, 1714, 1725, 1731, 1736, 1742, 1753, 1759, 1764, 1770, 1781, 1787, 1792, 1798, 1804, 1810, 1821, 1827, 1832, 1838, 1849, 1855, 1860, 1866, 1877, 1883, 1888, 1894, 1900, 1906, 1917, 1923, 1928, 1934, 1945, 1951, 1956, 1962, 1973, 1979, 1984, 1990, 2001, 2007, and 2012.

As the two sets are clearly distinct (as my Wedding Anniversary cannot fall on both a Friday and a Sunday in the same year), we can simply add the 12 + 63 = 75 years that meet our criteria.

Thanks to everyone that submitted a solution to this past week’s puzzle, and congratulations to Danny Bakker for submitting a correct responses and being drawn at random from the plethora of correct responses submitted. A $50 Gift Certificate from our puzzle sponsor ThinkGeek will be on its way to Danny shortly.

Happy puzzling!

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