Towel Day is celebrated every year on 25 May as a tribute to author Douglas Adams
On this day, fans openly carry a towel with them, as described in Douglas Adams’ The Hitchhikers Guide to the Galaxy, to demonstrate their appreciation for the books and the author. The commemoration was first held 25 May 2001, two weeks after Adams death on 11 May. Since then it has become a yearly celebration.
On Towel Day 2015, astronaut Samantha Cristoforetti sent a “Towel Day greeting” and read aloud a sample from The Hitchhiker’s Guide To The Galaxy from the International Space Station. In 2016 Briton Tim Peake celebrated Towel Day in space.
Source: Wikipedia
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