It's getting trendy to celebrate Pi Day on March 14 (3/14) of every year, and in a couple of years we'll even have a couple of extra digits: 3/14/15. Then a year after that we get the rounded version, 3/14/16.

Recently we had a Fibonacci Day: 5/8/13. The Fibonacci Sequence is the sequence of numbers starting 1,1 and then each following number is the sum of the previous two: 1,1,2,3,5,8,13,21... Given that the first number of an American-style date can't be greater than 12, there are only six Fibonacci Days each century: 1/1/02, 1/2/03, 2/3/05, 3/5/08, 5/8/13 and 8/13/21. So we get only one more this century. If you write dates European-style (DD/MM/YY), you don't get 8/13/21, because there's no 13th month, plus four of the other five fall on different days than if you do American-style dates. For example, the second Fibonacci Day would be February 1, '03 instead of January 2, '03.

Tomorrow could be called a Pythagoras Day, because it's 5/12/13 and the three numbers satisfy the Pythagorean Theorem, i.e. square and add the first two, and you get the square of the third. In other words, the three numbers are the side lengths of a right triangle. The other Pythagoras Days are...get ready...3/4/05, 8/15/17 and 7/24/25. That's it. If you write dates European-style, you only get 3/4/05 and 5/12/13, because the second number can't exceed 12.

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