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 referenced in Adams's The Hitchhiker's Guide to the Galaxy. The commemoration was first held in 2001, two weeks after Adams's death on 11 May 2001.I forget my towel, so I post this Linus image: don't panic!
I search on Scholar The Hitchhiker's Guide to the Galaxy and I find some references
The Hitchhiker's Guide To Teaching Legal Research To The Google Generation (pdf)
A guide to the Guide by Cohen-Rose and Christiansen (Google Books preview)
Equation-of-Motion Coupled-Cluster Methods for Open-Shell and Electronically Excited Species: The Hitchhiker's Guide to Fock Spacez by Anna I. Krylov
And there's also a ref. in Penrose's book The Road to Reality: A Complete Guide to the Laws of the Universe
I conclude with a video from the official site of Towel Day