Why Pi Day 2016 Is Extra-Special A date like this one won't come around for another 100 years.

 -- Math nerds likely already circled today on the calendar, but it's worth noting this year's Pi Day is a once-in-a-century occurrence.

March 14 — 3/14 — celebrates the mathematical constant pi, the ratio of circumference of a circle divided by its diameter. While it is often abbreviated as 3.14, pi has an infinite number of digits beyond the decimal point, starting with 3.141592653.

Last year's Pi Day was one to celebrate, since it was 3/14/15, matching pi's first five digits. Last year hard-core math fans started celebrating the day at 9:26:53 in the morning. There's a big reason to celebrate this year too: Math enthusiasts are calling today Rounded Pi Day.

Pi rounded to the ten-thousandth (four digits past the decimal point) is 3.1416, matching today's date, 3/14/16.

And if you need any more reason to geek out about March 14 this year, here's one: Albert Einstein was born 137 years ago today.