The Pi day that happens once every hundred years. (unless you are using the Japanese calendar, when it happens more often)
Pi is an irrational number, which means that it cannot be expressed as a decimal number, nor can it be expressed as a fraction. Pi is the ratio of a circle's diameter and circumference. It is an important and significant number for many calculations, yet it cannot be calculated fully. The circle is one of the basic and important geometric shapes, and is used in many ways. The circle is the most efficient use of border for internal area, and the closer something gets to a circle, the more internal area it has for the same border distance. Pi is how
many times longer the circumference of a circle is than the diameter, and Pi is used to calculate the area of a circle, as well as the volume of a cylinder, sphere, or cone, which are common shapes used in our world, such as with plumbing. Pi can be visualised with physical things, yet can only be approximated for calculations.
Because the decimal numbers of Pi go on forever without a repeating pattern, it is impossible to ever express Pi numerically, so the number is rounded off to an approximation.
Therefore, Pi day is March (3) 14th, 2015 (15), 9:26, Saturday (S, or 5), to form the numbers 3.14159265
(Saturday can also be abbreviated as Sa, which can also be represented as 54 [because 5 looks like S and 4 looks like a], and Pi rounded off to 9 decimal places is 3.141592654)
Happy Pi Day!