Pi is the ratio of the circumference of a circle to its diameter. Regardless of the size of the circle, pi is always the same irrational number: approximately 3.14 or 22 divided by 7. Because July 22 is 22/7 in the date format used by most of the world, it is celebrated as Pi Approximation Day. March 14 (3.14 in a US month-first date format) is celebrated as Pi Day primarily in the US.
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