March 14th, when written in the US format with the month before the day, is 3.14 — which makes last Friday Pi Day!

To belatedly celebrate this momentous day, here are some of the articles about Pi we have featured on Plus:

• Mathematical mysteries: Transcendental meditation — We make rational numbers from integers by allowing division by integers other than zero. Rational numbers were all the Greeks allowed. This left them confused — and sometimes frightened — when geometric results such as Pythagoras' Theorem seemed to imply that rational numbers weren't enough. And what to do with Pi?
• Remembrance of numbers past — In March 2004, Daniel Tammet from Kent set a new European record when he recited Pi from memory to 22,511 decimal places. It took him five hours to complete the task, yet he had barely made it halfway to the world record of 42,195 digits set by Hiroyuki Goto of Japan in 1995.
• Pi not a piece of cake — Ever since the Egyptians' first attempts to calculate Pi over two millennia ago, the number has been a constant in the minds of mathematicians.
• Pushing back Pi — Numbers like Pi have no repeating pattern. So just how accurately do we know what it is?
• What is the Area of a Circle? — And what's Pi got to do with it?

Pi Day also happily happens to be Albert Einstein's birthday. We can now look forward to various Pi Approximation Days:

• July 22: 22/7, a common approximation of Pi
• November 10: The 314th day of the year
• December 21, 1:13 p.m.: The 355th day of the year, celebrated at 1:13 pm for the Chinese approximation 355/11