In 1975 a movie (narrated by Rod Serling) called The Outer Space Connection, based on the book, was released. The book claimed that the aliens would return to earth on December 24, 2011. Oh look, that's today! A July 29, 1975 Redlands Daily Facts (Redlands, CA) article about the release of the movie appears below.
[UPDATE: I obviously read the story wrong initially. It's just The Outer Space Connection, not Chariot of the Gods? that cites that date.]