Pi Day and Pi Approximation Day are two unofficial holidays held to celebrate the mathematical constant π (Pi). Pi Day is observed on March 14; Pi Approximation Day may be observed on any of several dates, most often July 22 (22/7 - in European date format - is a popular approximation of π). It is not clear how many people know about the holiday or celebrate it.
It's unclear when Pi Day was first celebrated. The Exploratorium began an annual public celebration of Pi Day in 1987 or 1988, with staff and public marching around one of its circular spaces, and then consuming fruit pies; the museum has since added pizza pies to its Pi Day menu. Enthusiasts also note that the day happens to be Albert Einstein's birthday, among other famous birthdays on this day. Massachusetts Institute of Technology, known for its sometimes unconventional and quirky take on mathematics, often mails out its acceptance letters to be delivered to prospective students on Pi Day.
Celebrate all things circular today.