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Towel Day is celebrated every 25 May as a tribute by fans of the late author Douglas Adams. On this day, fans carry a towel with them to demonstrate their love for the books and the author, as referred to in Adams' The Hitchhiker's Guide to the Galaxy. The commemoration was first held in 2001, two weeks after Adams' death on 11 May 2001.

http://en.wikipedia.org/wiki/Towel_day