Towel Day is an annual celebration on the 25th of May, as a tribute to the late author Douglas Adams (1952-2001). On that day, fans around the universe proudly carry a towel in his honour.

From towelday.org. According to The Hitchhiker’s Guide to the Galaxy:

A towel, it says, is about the most massively useful thing an interstellar hitch hiker 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 â…¤, 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 mindbogglingly 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.

So… Carry your towel proudly and avoid Vogon poetry.