25 May (the day his memorial took place after his death on 11 May 2001). To recognise Adams' invaluable contribution to the fine art of interstellar travel and the necessity to make the towel ubiquitous to travelling.
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 vapours; you can sleep under it beneath the stars which shine so redly on the desert world of Kakrafoon; use it to sail a mini raft 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 to avoid the gaze of the Ravenous Bugblatter Beast of Traal (a mindboggingly 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 [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 hitchhiker any of these or a dozen other items that the hitchhiker 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."
Source: The Hitchhiker's Guide to the Galaxy, Chapter Three
Trivia about Douglas Adams Towel Day
• Adams was cremated with his towel
• Douglas Adams Towel Day appears on Fodor's firm calendar