The early 19th century saw a great religious revival within the United States referred to as the Great Awakening.
The message was a simplified one of spiritual redemption for the individual. The primary difference of their belief system from many of today’s evangelical beliefs is that they felt that the Kingdom of God would occur before the return of Jesus. Therefore the reform of society and politics was very much in the forefront of their agenda.
William Miller (1782–1849), preached a prophetic message based particularly on the prophetic books of Daniel and the Revelation to John. Based on an interpretation of Daniel 8:14, which speaks of 2,300 days, he concluded that Christ would return about 1843.
The movement became a multi-denomination national one. That we speak of Millerism in the past tense is referred to as the Great Disappointment.
The movement is the fore runner to the Seven Day Adventists. It’s geographical basis of Upstate New York also makes them close neighbors of Joseph Smith who was slightly ahead of the Millerites in the time line of developing the Church of the Latter Day Saints.
In a more modern time frame the Branch Dividians were a Adventist offshoot movement.