Invented holidays are nothing new. The greeting card industry has been doing it for years. But here's one you may not be aware of, and it wasn't the greeting card industry that created it. It was created by a physicist back in 1989.

Pi Day is held every March 14th to commemorate the mathematical constant of Pi (π), because of the date's representation as 3/14 in month/day date format. This is because of the commonly used approximation of 3.14 for π. It was created by Larry Shaw back in 1989 when he worked as a physicist at the San Francisco Exploratorium, where Pi Day is still celebrated with the staff marching around and eating fruit pies. Since the first Pi Day, others have celebrated the holiday in some pretty unique ways.

In 2004, Daniel Tammant, who is a high functioning autistic savant and British writer, recited Pi to 22,514 places. And in 2009, The U.S. House of Representatives, passed a non binding resolution recognizing March 14th as National Pi Day.

In the last few years, it has also become something of a geek holiday as well. In 2010, Google did one of it's Google Doodles for Pi Day, with the word Google laid over images of circles and pi symbols. And the Massachusetts Institute of Technology mails it's application decision letters on Pi Day.

The traditional way to celebrate Pi Day is eat pies (because pie and pi are homophones) and to discuss the relavance of pi. I will be enjoying a nice apple pie with some vanilla ice cream. How will you celebrate Pi Day?