Image Credit: Timothy Muza
Pi is the mathematical constant that determines the ratio of the circumference of a circle to it's diameter, and the area of a circle to it's radius. It is represented by the greek letter p.
Pi Day is celebrated on 14th March each year.
Pi's value is usually taken to be 3.14, hence March 14th, March being the third month.
So basically, it is a gag on the date.
To celebrate the day, you can:-
The official website of Pi Day can be found at piday.org, where you can find out more about the day and even see Pi calculated to 1 million digits.
If your kids are struggling with using Pi to find the area and circumference of circles, there is a good explanation on BBC Bitesize, with a video and practice questions.