Happy Pi Day! On March 14, mathematicians, scientists and everyone else who just loves pi (or pie) celebrate Pi Day—so named for the Greek symbol π, the ratio of the circumference of a circle to its diameter. The number’s first three digits are 3.14, so its holiday (which was officially designated by the House of Representatives in 2009) falls on 3/14.

Pi has been around for quite a while. Although the Babylonians and Egyptians gave estimates of the number nearly 4,000 years ago, the first accurate calculation was conducted by Archimedes of Syracuse around 250 BC. The famous mathematician used the Pythagorean Theorem to find that the number was between 3 1/7 and 3 10/71. But mathematicians didn't start using the symbol π until 1706, when the English mathematician William Jones used it to indicate the number.

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And if you really want to get technical, you can celebrate Pi Minute (3/14 at 1:59), or even Pi Second (3/14 at 1:59:26). You could theoretically extend this infinitely --π is an irrational number, which means that it can't be represented by a fraction of two integers, and it extends infinitely with non-repeating digits.

Pi Day began in 1988 at the San Francisco Exploratorium, which is holding its 25th annual Pi Day today. Other groups across the country are celebrating, too—in Princeton, New Jersey, both Pi Day and Albert Einstein’s birthday (which is also on March 14) are honored with pie-eating contests and Einstein lookalike competitions. Also, MIT usually mails out its admission decisions on Pi Day.

But you don’t need much to have a great Pi Day—check out the videos below for some of the greatest tributes to this holiday: