Have a Great Pi Day on March 14. What is Pi Day & What Are Some Ways to Celebrate Pi Day? Pi Day is an annual celebration held on March 14th (3/14) to honor the mathematical constant π (pi). Pi is the ratio of the circumference of a circle to its diameter, and it is an irrational number that goes on infinitely without repeating. Pi is a fundamental concept in mathematics and is used in a wide range of applications, from geometry and trigonometry to physics and engineering.Pi Day was first celebrated in 1988 by Larry Shaw, a physicist at the Exploratorium in San Francisco, California. The idea was to celebrate pi and promote interest in math and science among the…