"A towel ... is about the most massively useful thing an interstellar hitchhiker can have." And so say all of us! Happy Towel day 2012.

On 25 May, fans of Douglas Adams pay tribute to the man and his work by carrying a towel around with them all day in honour of The Hitchhiker's Guide to the Galaxy's wise words on the importance of having a towel always about one's person. The practical benefits are obvious to anyone who has ever bounded across the cold moons of Jaglan Beta, or basked on the beaches of Santraginus V.

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.

This annual celebration started after Adams' death in 2001 and is observed by readers across the world. There's a wealth of great photos uploaded to Flickr of fans with their towels, and we've gathered a few together in this Flickr gallery.

We on the books desk are of course celebrating this day with our towels draped around our shoulders. We're encouraging Twitter followers to tweet us their Towel day photos, and we invite you to use this open thread to post your favourite lines from any Douglas Adams books.

All that remains is for us to ask, do you know where your towel is?