Wikipedia said:Towel Day is celebrated every year on 25 May as a tribute to the author Douglas Adams by his fans.[1] On this day, fans carry a towel with them to demonstrate their appreciation 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.[2]
I think it may be connected with the other new achievement, The Hitchhiker's Guide to the Prairie, for the quest line "Attack of the Wogons".
If it is not related to this new quest line, perhaps it is connected with some activity we need to do while wearing another set of clothes that we attain from the Union Pacific bond shop.