Pi is an irrational number, and thus cannot be completely represented to its final digit. However, 3.14 is frequently used in formulas to represent pi, thus the "naming" of March 14 as Pi Day. 3.1415926535 is pi to a decent number of digits, but computers have calculated pi to over 1 trillion digits past the decimal. Pi represents the relationship between a circle’s diameter and its circumference.
Pi Day is really a one of those pseudo-holidays, such as "Talk Like a Pirate Day." Larry Shaw (above), who worked as a physicist at the San Francisco Exploratorium, created Pi Day in 1989. That first event was held with both the public and staff joining the activities, marching around a circular space in the building, which makes sense considering pi's relationship to circles. The party then moved on to eating, with participants then feasting on fruit pies.
The Exploratorium continues to hold Pi Day celebrations.
The date has become so well-known that there are numerous Pi Day websites. Among them are what is likely the official site, at piday.org. That one is being hammered to the point of being not usable, though.
In addition, the Exploratorium has its own Pi Day website. Princeton does as well. Ironically enough, March 14 is also the birthday of famed physicist Albert Einstein.
On March 12, 2009, the U.S. House of Representatives passed a non-binding resolution (HRES 224), which recognized March 14, 2009, as National Pi Day. Meanwhile, on Pi Day 2010, Google used one of its familiar Google Doodles to celebrate the faux holiday.
Pi Day 2015 will be an especially significant day for pi, and perhaps pie, enthusiasts. That day, 3/14/15, will represent the first five digits of pi, 3.1415.
Sadly, it doesn't appear restaurants like Baker's Square will be celebrating Pi Day. After all, there was already a National Pie Day (with an e) earlier this year, on Jan. 23.