Who: Dorothy Martin, a Chicago housewife and student of Dianetics, a set of practices developed by science fiction author L. Ron Hubbard.
How she came by this date: Through automatic writing, Martin came in contact with beings from the planet Clarion, who told her that the world would be destroyed by flood and that the faithful would be rescued at midnight by flying saucers (or so she said).
What actually happened: Martin's followers, many of whom quit their jobs and gave away their possessions, gathered in her home to await the aliens. (Martin's husband, a nonbeliever, slept upstairs through the whole thing.) To avoid being burned by the flying saucer, her followers removed all metal from their persons, including zippers and bra straps. Midnight came and went and the group became increasingly agitated. Finally, at 4:45am, Martin said that she received another message from Clarions informing her that God was so impressed by her groups actions that He changed His mind and decided to spare the earth.
The group was infiltrated by a psychologist named Leon Festinger, who used his observations to develop the theory of cognitive dissonance.