Happy what day? Tau is a mathematical constant, whose value is 6.28... . Is that ringing any bells? Correct:

OK, so tau being 6.28 explains why 28th June is Tau Day (blame the Americans - they write their dates backwards). But why do we need a new mathematical constant, especially one that is simply double another one?

Some maths teachers and academics have been in favour of using tau instead of pi in maths teaching, particularly in early years. It's because 2 pi seems to crop up a lot, probably more often than a single pi, particularly in geometry and trigonometry. For example, the circumference of a circle is given by:

And if we went to a system where both constants were in use, would this not just add to the confusion, rather than alleviate it?

Finally there are thousands of years of pi tradition. The ancient Greeks obsessed over pi, just as much as modern mathematicians do.

What do you think of tau?