In the mid-1920’s, one Joseph Hennessy was executed for attempting to assassinate Calvin Coolidge, but the husband of one of his descendants is eager to show how a letter dated only “Friday the 13th” might prove his innocence.

This is a calendar mystery such as Asimov occasionally indulges in, one that deals with the actual pattern of leap years in the Gregorian calendar. That pattern is relatively straightforward, but it was nonetheless something I found myself explaining increasingly often as the year 2000 approached. It’s kind of a sticky point, but something a reasonably knowledgable person could work out by pencil and paper before reading Henry’s solution. On the whole, this is a pleasant story.

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