Today is 3-14-15, which means March 14, 2015. This also corresponds with the first five digits of pi, the infinite number, 3.1415. Hence, the day today is titled as Pi Day, but not just any Pi Day. In fact, it is Pi Day of the Century.

You might say that’s irrational. Mathematicians would agree.

And therein lies the delight of Pi Day.

You probably remember pi from grade school, but here’s a quick refresher: Pi is the number that approximates the ratio of a circle’s circumference to its diameter. In other words, if you divide the circumference of any circle by its diameter, you always get pi, an irrational number that starts with 3.1415 and goes on forever. It shares its name with the 16th letter of the Greek alphabet.

The idea of celebrating Pi Day on March 14 — coincidentally, Albert Einstein’s birthday — started in the 1980s. The first large-scale celebration was organized in 1988 at the Exploratorium museum in San Francisco, and Pi Day got a boost two decades later when the U.S. House of Representatives passed a resolution supporting its celebration in 2009.

“It’s mind-blowing to think about a number that’s infinite and non-repeating,” said Dante Centuori, director of creative productions at Cleveland’s Great Lakes Science Center. Add to that the fact that pi sounds like pie, and you have a sweet connection the ancient Greeks couldn’t possibly have dreamed up, Centuori said.

Some just couldn’t wait for the big day. The members of the Undergraduate Mathematics Club at Kent State University celebrated at its biweekly meeting earlier this week with activities that included a talk on the history of pi, a debate on the merits of pi vs. tau (that’s 2 times pi, for you English majors) and a beauty contest comparing the complexity of the various fractions and formulas used to approximate pi.

Club secretary Courtney Augustine said before the meeting that the members would also be guessing the circumference of the pizzas being served.