Pi day Today is the 14th of March of 2015. If we write it in American date format we will get 3/14/15. It looks exactly as first 5 digits of Pi - 3.1415. That's why today is called Pi day. And it's going to be the only Pi day in our whole timeline. In case you wonder what is Pi - it's the one of most important numbers in the Universe. It appears in many applications and many formulas include Pi as it's part. Pi is the number which defines the length of a circle divided by it's diameter. If you try to calculate it yourself you will find that this number doesn't depend on size of the circle and is always equal to 3.141592... It's irrational number which means that you will never stop trying to write it down, it goes on and on. It also doesn't contain any repetitions inside it. There is another remarkable irrational number out there in the Universe - e, I will discuss it later. Happy Pi day, everyone!