Happy Pi Day 2016! Pi Day is an annual celebration of the mathematical constant of pi in the United States. It's observed on March 14 because 3, 1, and 4 are the first three numbers in pi. According to Scientific America, only the United States and Belize can claim "Pi Day" as theirs, because "most of the rest of the world relies on the day-month-year format, which makes more sense logically because the date elements go from small to big. But this dd-mm-yy format would have pi day fall on the third day of the 14th month of the year." Click on for funny memes and Pi Day facts provided by the official Pi Day website.