Why 'Pi Day' 2016 Is Extra Special

PHOTO: The "Pi" symbol, shown in this undated photo, is a mathematical constant that is the ratio of a circles circumference to its diameter.PlayGetty Images
WATCH Solve Pi Day Puzzle for 3.14 Years of Free Pizza

Math nerds likely had 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 -- or 3/14 -- celebrates the mathematical constant of pi. Pi represents 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, perfectly matching the first numbers past the decimal point of pi. Last year, hardcore math fans even started celebrating the day at exactly 9:26 a.m. and 53 seconds. There's a big reason to celebrate this year too -- math enthusiasts are calling today "Rounded Pi Day."

When rounding pi to the ten-thousandth (that's four numbers past the decimal point), it comes out to 3.1416, matching today's date -- March 14, 2016.

And if you need any more reason to geek out about March 14, here's one: it's Albert Einstein's 137th birthday.