When the number Pi (p) is expressed as a decimal, it's written as 3.14. So we came up with Pi Day, which we celebrate on the 14th day of the 3rd month every year (3/14). Really it's just an excuse for kids at school to eat pie during their math lesson.
This year's Pi Day is a little more exciting though. The number Pi isn't just 3.14. It's decimal expansion is never ending. And the next two numbers are 1 and 5. So this year, 2015, the date lines up perfectly. Pi is 3.1415 and Pi Day will be 3/14/15.