The Hitchhiker's Guide to the Galaxy (1979)

In-fiction date: 5/25

May 25th is Towel Day, observed worldwide by fans of Douglas Adams since 2001, two weeks after his death. The towel earns its place because the Guide devotes considerable attention to its virtues: 'A towel is about the most massively useful thing an interstellar hitchhiker can have.' It also happens to be a Thursday — the day of the week on which the Vogon constructor fleet appears in Earth's sky and demolishes the planet to make way for a hyperspace bypass, while Arthur Dent is lying in front of a bulldozer in Cottington having a bad morning that is about to become significantly worse.