What could be more American than pie and puns based on the world’s only sane date-writing system? Put them together and you get today—Pi (π) day—which is to say, 3.14 (March 14). Most of the world doesn’t get our month-day format, just as most of the world doesn’t feature pies in their cuisine (at least, not proper pies). Which is fine, because that leaves more pie for us.
Source: Interesting Thing of the Day