Towel Day is celebrated every year on 25 May as a tribute to the author Douglas Adams by his fans. On this day, fans openly carry a towel with them, as described in Adams' The Hitchhiker's Guide to the Galaxy, to demonstrate their appreciation for the books and the author.

RIP Douglas Adams, you creatively smart bugger.

Of course, this meshes well with my sense of absurdist humour.

Yes, I was one of the original hoopy froods who knew where his towel was, after discovering the radio series and books way back in High School in 1980... gagh! They are still one of my favourite series of comedic books around. I adore the absurd abstractions on philosophy, theology and space/time in general! Thank you, Douglas Adams, for enabling us to take a 90-degree twist on reality with these books.

I have all six books in the (increasingly inaccurately named) Trilogy... I bought myself a copy of the sixth book a year ago.

Note: The Books and Radio Series, NOT the travesty of the 2005 film!

(Source)

As The Guide says:

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.)

- Douglas Adams, The Hitchhiker's Guide to the Galaxy

"Anything that happens, happens.
Anything that, in happening, causes something else to happen, causes something else to happen.
Anything that, in happening, causes itself to happen again, happens again.
It doesn't necessarily do it in chronological order, though."

- Douglas Adams, The Hitchhiker's Guide to the Galaxy