The Second coming would occur on April 28 or at the end of 1843

William Miller : religious leader
Although it was not officially endorsed by their leadership, many Millerites expected the Second Coming to occur on April 28 or at the end of 1843.
After Christ did not return on March 21, 1844, the Millerites then revised William Miller's prediction to October 22, 1844, claiming to have miscalculated Scripture. The realization that the predictions were incorrect resulted in the Great Disappointment.
The Millerites were the followers of the teachings of William Miller who, in 1833, first shared publicly his belief in the coming Second Advent of Jesus Christ in roughly the year 1843.

Leon Festinger et al., When Prophecy Fails. University of Minnesota Press, 1956.