Towel Day is celebrated every year on the 25th May as a tribute to the late author Douglas Adams by his fans. The Hitchhikers Guide to the Galaxy has this to say about towels:

A towel, it says, is about the most massively useful thing an interstellar hitchhiker can have. Partly it has great practical value. You can wrap it around you for warmth as you bound across the cold moons of Jaglan Beta; you can lie on it on the brilliant marble-sanded beaches of Santraginus V, inhaling the heady sea vapours; you can sleep under it beneath the stars which shine so redly on the desert world of Kakrafoon; use it to sail a miniraft down the slow heavy River Moth; wet it for use in hand-to-hand-combat; wrap it round your head to ward off noxious fumes or avoid the gaze of the Ravenous Bugblatter Beast of Traal (such a mind-bogglingly stupid animal, it assumes that if you can’t see it, it can’t see you); you can wave your towel in emergencies as a distress signal, and of course dry yourself off with it if it still seems to be clean enough.

On the 25th of May fans carry a towel with them. The commemoration was first held in 2001, two weeks after Adams’ death on 11th May.

Kinokuniya in George St, Sydney will be part of the worldwide celebration by offering customers who wear a towel 20% off on their full price book purchase. There are some conditions (of the sort Douglas Adams would have poked ridicule at):

To receive the discount, your towel must be around your neck when you approach the cashier staff.

Your towel should preferably be a bathroom towel or large hand towel.

One discount per towel and one discount per person.

Not available in conjunction with any other offer.

By the way, it may or may not be important not to confuse Towel Day with Geek Pride Day which is held on the same date.