(actually updated and slightly revised this 5/25/20!)
The Hitchhiker’s Guide to the Galaxy has a few things to say on the subject of 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 vapors; 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.
More importantly, a towel has immense psychological value. For some reason, if a strag (strag: non-hitch hiker) discovers that a hitch hiker 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.)
Towel Day, for the few of who don’t know, is the annual celebration of Douglas Adams’ life and work. It was first held two weeks after his death, fans were to carry a towel with them for the day to use as a talking point to encourage those who have never read HHGTTG to do so, or to just converse with someone about Adams. Adams is one of that handful of authors that I can’t imagine I’d be the same without having encountered/read/re-read/re-re-re-re-read, and so I do my best to pay a little tribute to him each year, even if it’s just carrying around a towel.
One of my long-delayed goals is to write up a good all-purpose Tribute to Douglas Adams post, and another Towel Day has come without me doing so. Belgium.
Next year . . . or later. (he says for at least the 5th straight year, a work ethic I like to believe Adams would recognize).
In the meantime, here’s some of what I’ve written about Adams. A couple of years back, I did a re-read of all of Adams’ (completed) fiction. For reasons beyond my ken (or recollection), I didn’t get around to blogging about the Dirk Gently books, but I did do the Hitchhiker’s Trilogy:
The Hitchhiker’s Guide to the Galaxy
The Restaurant at the End of the Universe
Life, The Universe and Everything
So Long, and Thanks For All The Fish
Mostly Harmless
Also, I should mention the one book Adams/Hitchhiker’s aficionado needs to read is Don’t Panic by Neil Gaiman, David K. Dickson and MJ Simpson.
I’ve only been able to get one of my sons into Adams, he’s the taller, thinner one in the picture from a few years ago. 
TowelDay.org is the best collection of resources on the day, recently posted this pretty cool video, shot on the ISS by astronaut Samantha Cristoforetti.
Even better—here’s an appearance by Douglas Adams himself from the old Letterman show—so glad someone preserved this:
Love the anecdote (Also, I want this tie.)
The readers of this blog, the authors who’ve corresponded with me/provided books for me to read/encouraged me—even promoted this here project.