Between 1831 and 1844, William Miller, a Baptist preacher, launched what he called the "great second advent awakening", also known as the Millerite Movement. Miller preached a set of fourteen rules for the interpretation of the Bible, which spread to followers throughout the world. Based on his study of the prophecy of Daniel 8:14, Miller calculated that Jesus would return to Earth sometime between 1843 and 1844. Others within the movement calculated a specific date of October 22, 1844.
When Jesus did not appear, Miller's followers experienced what came to be called "the Great Disappointment". Most of the thousands of followers left the movement. A few, however, went back to their Bibles to find why they had been disappointed. They concluded that the prophecy predicted not that Jesus would return to earth in 1844, but that a special ministry in heaven would be formed on that date. Miller continued to wait for the second coming and died in 1849.
Seventh-day Adventists maintain that Christ went into the second apartment of the heavenly sanctuary on October 22, 1844 to begin the investigative judgment of both righteous and wicked to see who is actually ready to go to heaven. This investigative judgment takes place prior to his actual return to earth, which they believe to be very soon.
Members of the Bahá'í Faith believe that the Great Disappointment wasn't a disappointment at all, and that the prophecy was fulfilled by the coming of the Báb, who heralded the coming of the return of Christ, Bahá'u'lláh. October 1844 is when the Báb started to make his public claim and began the Bábí religion.
Psychologists see this event as an example of cognitive dissonance.