March 14, or 3/14, is known as “Pi Day” because of its resemblance to the first three digits in the decimal expansion of $pi$ (pi), which is defined as the ratio of a circle's circumference to its diameter: The true circle constant is the ratio of a circle's circumference to its radius, not to its diameter.
Why do they call it PI?
It was first called "pi" in 1706 by [the Welsh mathematician] William Jones, because pi is the first letter in the Greek word perimitros, which means "perimeter." It's a special number.
What is the meaning of 2pi?
Euler is the first person who started to use radian in trigonometry. The circumference of a unit circle is 2pi, therefore he defined 360 degrees be 2pi radian. A radian is the measure of an angle subtended at the centre of a circle by an arc equal in length to the radian.