http://en.wikipedia.org/wiki/Towel_Day

"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 — daft as a brush, but very very ravenous); 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.

More importantly, a towel has immense psychological value. For some reason, if a strag (strag: non-hitch hiker) discovers that a hitchhiker has his towel with him, he will automatically assume that he is also in possession of a toothbrush, face flannel, soap, tin of biscuits, flask, compass, map, ball of string, gnat spray, wet weather gear, space suit etc., etc. Furthermore, the strag will then happily lend the hitch hiker any of these or a dozen other items that the hitch hiker might accidentally have "lost." What the strag will think is that any man who can hitch the length and breadth of the galaxy, rough it, slum it, struggle against terrible odds, win through, and still knows where his towel is, is clearly a man to be reckoned with.

Hence a phrase that has passed into hitchhiking slang, as in "Hey, you sass that hoopy Ford Prefect? There's a frood who really knows where his towel is." (Sass: know, be aware of, meet, have sex with; hoopy: really together guy; frood: really amazingly together guy."

Hitchhiker's Guide to the Galaxy

Every major city has things that fans of Douglas Adams
could do on Towel Day. Here in Melbourne you could have:

1) Gotten back to nature at the Royal Botanic Gardens.

2) Seen an exhibition at the National Gallery Victoria.

3) Taken a drive down the Great Ocean Road.

4) Perused the stalls at the Queen Victoria Market.

5) Explored the iconic laneway of
Degraves Street.

6) Spent the night at the Melbourne Zoo's "Roar 'n snore"
sleep over.

And so much more. Carrying your towel with you - the entire time
of course.

http://timeout.com/melbourne/things-to-do/things-to-do-in-melbourne-before-you-die

A towel is just great
For travelling amongst stars
They're even greater on Jupiter or Mars
Towels aren't just for the shower or the beach
They're great to take with you in skies beyond reach
Whose boundaries you breach.

"I resisted temptation for this declamation
To reach out to literary height
For high aspiration in such an oration
Would seem quite remarkably trite
So I thought something pithy and succinct and clever
Was exactly the right thing to write..."

http://theguardian.com/books/2014/mar/19/last-school-poems-douglas-adams-griff-rhys-jones

Apologies for the typo:

http://theguardian.com/books/2014/mar/19/lost-school-poems-douglas-adams-griff-rhys-jones

I first read the Hitch-Hikers Guide while on a plane to Melbourne. I'd read a revue in the National Times and bought the book at the airport.

When I got to the part where Zaphod Beeblebrox's craft was attacked by Magrathera with Eddie singing "When you walk through a storm", I was cackling so loudly that the stewardess asked me to to move to the empty back of the plane. I read that section each towel day to remind myself of a life highlight.