Towel Day, which takes place on the 25th of May, is an annual celebration of the life and work of comedy writer Douglas Adams.
Each year since his death (he died of a heart attack on 11th May 2001), comedy fans are encouraged to carry a towel around with them on the 25th May. It's a worldwide event, and in 2015 an astronaut on the International Space Station even got involved!
Why a towel? Those who have read, heard or watched the author's most famous work, The Hitchhiker's Guide To The Galaxy, may remember this passage in which The Guide says:
A towel 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-tohand-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 bush, but 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-hitchhiker) 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.
In previous years, special events organised to mark Towel Day have included a Vogon Poetry Slam, a party on the computer game Second Life, and meet ups in various bars around the world. The great website towelday.org collects up all the events each year, plus has more information about what Towel Day is all about, and more details on the life of Douglas Adams too.
The grave of Douglas Adams is in Highgate Cemetery in London. As can be seen from the picture below, fans often leave pens when they visit.