Towel Day - Celebrating the life and work of Douglas Adams
http://www.towelday.org/

the official site for towel day

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.

毛巾日

Ok, I saw this mentioned and I knew it had something to do with Hitchhiker's Guide to the Galaxy, but couldn't figure out exactly what.