The 25th of May is a day loaded with significance. It is Towel Day, a day celebrating the importance of towels, especially to those who are fans of Douglas Adams. In The Hitchhiker's Guide to the Galaxy it says:
"A towel, it says, is about the most massively useful thing an interstellar hitchhiker can have. Partly it has great practical value. You can wrap it around you for warmth as you bound across the cold moons of Jaglan Beta; you can lie on it on the brilliant marble-sanded beaches of Santraginus V, inhaling the heady sea vapours; you can sleep under it beneath the stars which shine so redly on the desert world of Kakrafoon; use it to sail a miniraft down the slow heavy River Moth; wet it for use in hand-to-hand-combat; wrap it round your head to ward off noxious fumes or avoid the gaze of the Ravenous Bugblatter Beast of Traal (such a mind-bogglingly stupid animal, it assumes that if you can't see it, it can't see you — daft as a brush, but very very ravenous); you can wave your towel in emergencies as a distress signal, and of course dry yourself off with it if it still seems to be clean enough."
It's also the anniversary of the Glorious Revolution in Ankh Morpork. "The 25th of May is quietly celebrated by the survivors of the People's Revolution, which ended the reign of Lord Winder. They wear a sprig of lilac and gather at the Small Gods Cemetery to honour the Watchmen who fell: Cecil Clapman, Ned Coates, Dai Dickins, John Keel, Horace Nancyball, Billy Wiglet, and (albeit temporarily) Reg Shoe. The slogan of the People's Revolution is "Truth, Justice, Freedom, Reasonably Priced Love, and a Hard-Boiled Egg!"
There is a huge cross over in the fandoms of Adams and Pratchett. Some people were having problems deciding which to go with, until some hoopy frood suggested lilac towels! A lilac towel casually thrown over one's shoulder will happily encompass both celebrations and bring to mind both great writers.