Happy PI Day! March 14th! National Pi Day on March 14th recognizes the mathematical constant π. Also known as pi, the first three and most recognized digits are 3.14. The day is celebrated by pi enthusiasts and pie lovers alike! Pi is the ratio between the circumference of a circle and its diameter. While the idea of pi has been known for nearly 4000 years, accurately calculating it has been something of slightly more recent mathematical development. Apple Pie!