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.
The true reason to celebrate Pi Day is that mathematics, which is a purely abstract subject, turns out to describe our universe so well. My book “The Big Bang of Numbers” explores how remarkably hardwired into our reality math is. Perhaps the most striking evidence comes from mathematical constants: those rare numbers, including pi, that break out of the pack by appearing so frequently — and often, unexpectedly — in natural phenomena and related equations, that mathematicians like me exalt them with special names and symbols.
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