Yes, 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 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.
(above from EpicPonyz, other pictures and quotes from the Wikipedia article linked at the top)
Towel Day: A Tribute to Douglas Adams
Monday May 14, 2001 06:00am PDT
Douglas Adams will be missed by his fans worldwide. So that all his fans everywhere can pay tribute to this genius, I propose that two weeks after his passing (May 25, 2001) be marked as "Towel Day". All Douglas Adams fans are encouraged to carry a towel with them for the day.
So long Douglas, and thanks for all the fish!– D Clyde Williamson, 2001-05-14
I had never heard of this commemoration before, so I'm not carrying my towel with me today. I do know exactly where it is, and I would have plenty of time to go grab it if a Vogon construction fleet were to show up... come to think of it, I'm sure Bill or another Interzone barista would let me use one of theirs in that event. So I'll just participate pictorially today. (Next from photobucket)
Thanks, Douglas Adams, for your hilarious philosophical meanderings that millions of us have shared.