According to Wikipedia, the first known celebration of Pi Day was organized at the San Francisco Exploratorium by Larry Shaw in 1988. In 2009, the United States House of Representatives passed a resolution recognizing March 14, 2009 as National Pi Day. Predictably, it has been monetized on a website and other places where T-shirts, coffee mugs and other pi-phernalia is available.
Archimedes (source of the pictorial hint on the puzzle's main page) is generally recognized as the first to propose a value for the constant. He used the 22/7 approximation that yields 3.1428571428, very close (through three significant figures) to its actual value of 3.1415926535. As an irrational number, there is no common fraction (ratio of whole numbers) corresponding to its actual value. π is not only irrational, but it is also transcendental. For more, click here or here; the former for general information and the latter if you want to memorize the first one million digits of the number.
For the musically-inclined math geek, there is this catchy tune.
GB adds: In a nice bit of synergy, we were able to share Steve's puzzle at the University of Minnesota Campus Club [click here for a scrumptious poster]; those submitting correct solutions were rewarded by a free slice of pie. Also, how many of you knew that Archimedes' visage appears on the front of the prestigious Fields Medal?
Elaborating on the π-inspired musical aspects, click here, here, and here for highly relevant material from Noam Elkies. Finally, click here for a 2015 New Yorker essay by Steven Strogatz on why pi matters.