March 14 is celebrated as Pi Day because the date, when written as 3/14, matches the start of the decimal expansion 3.14159… of the most famous mathematical constant.
By itself, pi is simply a number, one among countless others between 3 and 4. What makes it famous is that it’s built into every circle you see – circumference equals pi times diameter – not to mention a range of other, unrelated contexts in nature, from the bell curve distribution to general relativity.
