What was the 'Great Disappointment' and when did it occur?

The Great Disappointment describes a time in the history of Seventh-day Adventists when followers of William Miller (1782-1849), a licensed Baptist minister, became bitterly disenfranchised after his predictions for the second coming of Christ failed to eventuate in 1843 to 1844. These predictions were published in the book "Evidence from Scripture and History of the Second Coming of Christ, about the Year 1843" in 1836.

https://www.gotquestions.org/Great-Disappointment.html