Perhaps the best visual explanation of Pi:
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Pi Day is an unofficial holiday held in honor of the mathematical constant π (Pi). Pi Day is observed on March 14 and can be celebrated in a variety of ways. Parties or other observances may be held by mathematics departments in educational institutions. Harvard’s Math department, for instance, has a pi recitation contest as well as a pie eating contest. Mathematics or science clubs might gather to consider the role that the number ? has played in their lives and to imagine the world without ?. During such an event, pi celebrants may approximate ?, devise alternative values for ?, eat pie, play piÃ±ata, drink PiÃ±a Colada, eat pizza (which itself is called pizza pie), listen to the song “Pi” by Kate Bush, watch Pi, or recite Pi. The song 867-5309/Jenny is sometimes sung, replacing the digits with the first several digits of pi.
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.