Pi Day and Pi Approximation Day are two unofficial holidays held to celebrate the mathematical constant π (pi). Pi Day is celebrated on March 14, or in the month/day date format as 3/14; since 3, 1 and 4 are the three most significant digits of pi. March 14 is also the birthday of Albert Einstein so the two events are sometimes celebrated together. Pi Approximation Day is held on July 22, or in the more common day/month date format as 22/7, which is an approximate value of pi.

Pi Day is observed on March 14, because of the Ancient Greek mathematician Archimedes' first rough approximation of pi as being 3.14. (A few years later, Archimedes was able to calculate a much better approximation of pi.) However, 22/7 is actually a closer approximation of pi than 3.14 is. Thus, a more "accurate" Pi Day could be found in the more common calendar, 22/7, or July 22.

Sometimes the so-called Pi Minute is also commemorated. This one occurs twice on March 14 at 1:59 a.m., and 1:59 p.m. If pi is truncated to seven decimal places, it becomes 3.1415926, making the Pi Second occur on March 14 at 1:59:26 a.m. (or 1:59:26 p.m.). If a 24-hour clock is used, the Pi Second occurs just once yearly, on March 14 (3/14) at 1:59:26 in the morning.

There is a large variety of ways of celebrating Pi Day and most of them include eating pie and discussing the relevance of pi. Pi Day is often celebrated with pies, given that pi and pie are homophones. The first Pi Day celebration was held at the San Francisco Exploratorium in 1988, with staff and public marching around one of its circular spaces, then consuming fruit pies. The museum has since added pizza to its Pi Day menu. The founder of Pi Day was Larry Shaw, a now-retired physicist at the Exploratorium who still helps out with the celebrations. (From: Wikipedia)