March 14 is Pi Day. It is a day to celebrate the mathematical constant pi (π) and to eat lots of pie.

It is celebrated in countries that follow the month/day (m/dd) date format, because the digits in the date, March 14 or 3/14, are the first three digits of π (3.14). Pi Day was founded by Physicist Larry Shaw in 1988.

Pi Approximation Day

Because everyone should be able to enjoy a fun mathematical holiday, people in countries that follow the day/month (dd/m) date format honor pi on Pi Approximation Day. The date of Pi Approximation Day – July 22 – when written in the day/month format or 22/7 corresponds to the fraction (22/7) that pi is usually depicted as.

There are many other days during the year when one can honor pi. Some of these are:

• March 4: The day marks the passing of 14% of the 3rd month of the year.
• April 5: By this day, 3.14 months of the year have passed.
• November 10: The 314th day of the year (November 9 in leap years).

An Irrational Number

One of the oldest and the most recognized mathematical constant in the world, Pi (π) is the ratio of any circle’s circumference to its diameter. Its value is approximately equal to 3.14159265. It is an irrational number, which means it cannot be expressed as a ratio of whole numbers, and its decimal representation never ends or repeats.

In recent years, mathematicians have called for replacing pi by tau (τ) as a way to describe the relationship between a circle’s circumference and its radius. In order to spread the word about the advantages of tau over pi, mathematicians around the world celebrate Tau Day on June 28.