Preacher William Miller of Massachusetts predicted the world would end on this date. Obviously, the world does not end, but Miller's ideas lead to the creation of the Adventist churches in America.
Miller has based his theology on a literal reading of apocalyptic passages in the Bible. He has tried to calculate the date of the Second Coming by using those passages, especially those in Daniel.
His original calculation was for the event to occur sometime between 1843 and 1844 (October 22nd, 1844, at the latest), and he convinced a lot of people to follow him in awaiting the event. Today is the second date he singled out and it, too, is a disappointment.