Pi Day is celebrated every March 14th (3/14 in the United States). As years go on, the once-nerdy holiday is becoming more popular even among those people who don't normally ascribe themselves to such events.
The celebration is even beginning to develop its own traditions, including an obvious edible. Pi Day is quickly becoming associated with it's own "traditional" food - pies!
Math enthusiasts have celebrated the day for years, in tribute to one of the mathematical constants in the Universe - pi. The first Pi Day celebration took place on 14 March 1988, at the San Francisco Exploratorium.
As nearly everyone learned in grade school, pi is the ratio of the circumference of a circle (the distance around the object), divided by it's diameter (the length of the circle). Ancient Greek astronomers realized the number, roughly equal to 3.14, is irrational - it has no last digit. They proved this using a simple trick. Simple formulas can reveal whether the last digit of a number is odd or even. These formulas show the last digit of pi to be both odd and even. Since there is no such number, pi cannot have a last digit.
Schools are beginning to use Pi Day as an opportunity to focus lessons on math and science. Teachers remind students mathematics is used in adult life by engineers, scientists and in many other professions.
Even large corporations are beginning to celebrate the occasion. In 2012, aerospace giant Raytheon sent approximately 500 apple pies to all math and science teachers withing a 3.14 mile radius of their plants.
Restaurants around the country are offering specials on pies of all sorts, including pizza pie! The Exploratorium is offering classes on how to properly throw pizza dough.
Celebrations of Pi Day also often include tributes to Albert Einstein - it is his birthday on 14 March. The next two years will be extra-special Pi Days. In twelve months, the American-style date will be 3/14/15, which comes closer to the actual number, which begins 3.14159... If you round pi off to four decimal places, the number is exactly the date one year later, 3/14/16.
America is one of the few nations in the world with a convention of writing the month of a date first, followed by the day, as in 3/14. The good news for people in other parts of the world is that pi is almost exactly equal to 22/7, so you only need to wait for 22 July to eat some pi(e)!