PI DAY - 3.14

In the US dateformat month comes before day, so 14 March is written 3.14 - and it's therefore called PI day.

By conincidence this date also happens to be the date of birth of Albert Einstein in 1879.

While PI is a constant with an unlimited number of decimals for maximum accuracy, a more inaccurate number is sometimes used in simple math to get close to the real PI: 22/7.

22/7 is only accurate down to 2nd decimal, yet it also has it's own day: The PI Approximation Day on 22/7.