It's time to get irrational. Sunday March 14 is Pi Day, when mathematicians will gather to celebrate the mystery of science's most famous strange number — Pi.

Pi, or the Greek letter π, starts with 3.1415926535 . . . ad infinitum without repeating. It is the figure obtained when the circumference of a circle is divided by its diameter, and it cannot be expressed as a fraction, making it an irrational number. Computers have calculated it to more than one trillion digits past the decimal point.

March 14 has been celebrated as Pi Day for more than 20 years after Larry Shaw, a physicist at the San Francisco Exploratorium science exhibition, decided to start it as a "geek holiday". The event has snowballed into an international phenomenon, with Pi parties and educational events in many different countries.

The symbol for Pi was first used in 1706 by William Jones, but became popular after it was adopted by the Swiss mathematician Leonhard Euler in 1737. Pi has intrigued and exasperated mathematicians and scientists for milleniums — how can such a simple concept as the circle be so difficult to pin down? Pi goes beyond geometry, it is a basic tool in architecture and is used to calculate economic statistics and a wide range of other complex computations.

At the Exploratorium tomorrow at 1.59pm a parade of people will process approximately 3.14 times around a shrine to Pi — and then eat pie. The day is also celebrated in schools and the US Congress has recognised it officially as a way to encourage maths and science education.

For devotees, there are Pi plates for your piece of pie, Pi T-shirts and even a limerick competition at www.piday.org .

At Pi parties, people will compete to recite as many decimal places for Pi as possible. They will need to show some endurance if they are to beat the world record held by Lu Chao, a 24-year-old graduate student from China, who took 24 hours and 4 minutes to recite to the 67,890th decimal place of Pi without an error.

Members of the World Federation of Pi will be logging on to the website mathematicianspictures.com to watch a giant Pi symbol drop down like the crystal ball in Times Square on New Year's Eve.

Visitors to www.piday.org bear witness to the fervour Pi can engender in those who like numbers Summer commented: "It makes me so happy, the possiblitly of an infinite number inspires me. It makes me feel like my dreams are infinite. I love math, and Pi is a real inspiration."

Erin wrote: "Pi is a magical loophole in our assumed and intuitive structures."

Joe said: "It's yum."

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