Who: Samuel S. Snow, a preacher in the Millerite movement, led by the Baptist preacher William Miller
How he came by this date: A prophesy in the Book of Daniel states "Unto two thousand and three hundred days; then shall the sanctuary be cleansed" (Dan. 8:14). If you convert the days into years, and if you start in the year 457 BC – the year that Artaxerxes I of Persia decreed that that the city government of Jerusalem shall be re-established – then this takes you to 1844. Using the Karaite Jewish calendar, Snow pinned the date down to October 22.
What actually happened: Thousands of people gave away all their posessions, only to be surprised when the world did not come to end, and the day came to be known as "The Great Disappointment." The Millerites splintered into several religious groups, the largest and most mainstream being the Seventh Day Adventists, and the smallest and most unconventional probably being the Branch Davidians. Millerism has also influenced the Bahá'í Faith.