Pi Day is largely observed in the US on 14 March to celebrate the mathematical constant π (pi) since 3, 1, and 4 are the first three significant digits – which also represent the 14^{th}day of the third month. Celebrations of the day coincide with Albert Einstein's birthday.

**History of Pi Day**

Pi Day was first know to have been observed in 1988 when Physicist Larry Shaw at the San Francisco Exploratorium, led celebrations with staff and members of the public, who marched around in a circular space and ate fruit pies. The Exploratorium continues to hold Pi Day celebrations to this date.

In 2009, the US House of Representatives recognised 14 March as National Pi Day and passed a resolution saying: "Whereas mathematics and science can be a fun and interesting part of a child's education, and learning about pi can be an engaging way to teach children about geometry and attract them to study science and mathematics ... Pi can be approximated as 3.14, and thus March 14, 2009, is an appropriate day for 'National Pi Day'."

The 2015 version, or 3/14/15, was considered a super special Pi Day with the numbers configuring with the year as well as the day and month, an event that happens once a century.

**What is Pi?**

Pi is an irrational and transcendental number used in mathematics to represent a constant – the ratio of the circumference of a circle to its diameter – which is approximately 3.14159. Pi has been calculated to over one trillion digits beyond its decimal point although only a handful of digits are needed for typical calculations.

It's not equal to the ratio of any two whole numbers, so an approximation – 22/7 – is used in many calculations. The value of Pi is essential in architecture and construction based calculations and was used frequently by early astronomers. Pi has been known for about 4,000 years, but it started to be called by the Greek letter only in the 1700s.

**How Pi Day is celebrated?**

Pi Day can be celebrated in a number of ways, most commonly by eating pies, discussing its significance, holding math quizzes etc. Another popular activity is holding competitions to see who can recall it to the highest number of decimal places.

Nasa's Jet Propulsion Laboratory this year has launched its third Pi Day challenge, which gives students a chance to compete with its scientists and engineers. They use Pi to calculate how much sunlight is blocked by Mercury as it passes between Earth and the Sun, among other problems. While the four illustrated math programs are designed for kids between grades 4 and 12, others are welcome to give it a try as well.

Pizza Hut in the US this year is enlisting Princeton's Professor Emeritus John H Conway to concoct math problems that could give some lucky winners free pizza for 3.14 years. Visit the Pizza Hut blog at 8 am on Pi Day to check out the problems.