I love this one. Really, it's maths not physics, but there is a bit of experimental physics creeping in at the fringes when the experimenters realize that the first method is biased. The second method is much better designed.
Regrettably, pi-day (March 14th, 2015, or 3.14.15) only works if you use the US system of recording dates. But fear not, e-day (2nd July 2018, or February 7th, 2018 if you're American) isn't so far away…