What is Pi Day? The story of why some celebrate March 14 – and others say it’s a ‘powerful conspiracy’

14 March is celebrated by some as the most exciting day in mathematics – when the date lines up in the numbers of the famous constant, Pi or π. But some people would rather it isn’t celebrated at all.

The day looks like 3.14 for those that write dates in the month then day format, but that is rarely used outside of the Americas.

Many people instead opt to celebrate the potentially more interesting Pi approximation day, which takes place on 22 July or 22/7 – a way of working out an approximate value for Pi for use in rough calculations.

And still others would rather that Pi day was not celebrated at all, and say that the number should not be treated with such reverence. Instead, they say that we should use Tau – a number that serves much the same purpose and can be celebrated on 28 June, or 6/28 in America.

The complaints about the date are relatively obvious. Most places around the world use a different system to the US, where Pi Day was born.

By far the most popular date format is day/month/year, as is used in the UK as well as much of Asia, Australia, Africa and South America. It is not possible to write Pi day using that format, but it is easy to write Pi approximation day.

In China and many other countries in south-east Asia, dates are written year/month/day. It is possible to do Pi day in that format since the day (14) does follow the month (3), but the year comes first and throws that off slightly.

But the complaints about the competitor to Pi Day, Tau Day, are much more aggressive – and campaigners for the change have made a website and a full manifesto, laying out exactly why the world is wrong.

That manifesto, written by Tau fan Michael Hartl, is dedicated to the “true circle constant”, which he says should be referred to by the Greek letter Tau.

The number itself is simply Pi, but doubled – 6.28318, and so on. That makes it easier to use in many applications, campaigners claim.

But those behind the Tau manifesto admit that they are facing a difficult challenge: “a powerful conspiracy, centuries old, determined to propagate pro-Pi propaganda”.

But they hope that they can prove that Pi “is a confusing and unnatural choice for the circle constant”, they say, even if the name of the constant and its day doesn’t actually change.

To settle the argument, some people have taken to celebrating a specific “Tau time” on Pi day – 6.28pm. That is when, for instance, the Massachusetts Institute of Technology sends out its admissions decisions over the internet, which was a change made to welcome Tau into the celebrations.