Calculating the ratio of a circle's circumference to its diameter is as easy as pi. Today is Pi Day, an event that celebrates this mathematical constant. This date was chosen because 3, 1 and 4 are the first three significant figures of pi (π) when it is written out. Pi is an irrational number, which means that it can't be written as a fraction. When written out, its decimal representation goes on infinitely without repeating. The oldest attempts to calculate pi date back to ancient Babylon and Egypt, around 1,900 BCE.