The 14th of March in the US is written 3/14 and 3, 1 and 4 are the most significant digits of pi (π) i.e. 3.14. Though some people say that the real pi day won’t be until March 14th, 2015 at 9:26:53 AM when the date and time matches the first 10 digits of pi 3.141592653.
pi-day