Daniel 8:13, 14
Then I heard one saint speaking, and another saint said unto that certain [saint] which spake, How long [shall be] the vision [concerning] the daily [sacrifice], and the transgression of desolation, to give both the sanctuary and the host to be trodden under foot? And he said unto me, Unto two thousand and three hundred days; then shall the sanctuary be cleansed.
The 2,300 day/year prophecy of Daniel 8 has been found to end in 1844. At that time, according to Daniel 8:14, "then shall the sanctuary be cleansed". Beginning with the decree to rebuild Jerusalem in the Fall of 457 BC (keep in mind that there isn't a year "0") and adding 2,300 years you will end up at the Fall of 1844.
When it was first discovered that this prophecy ended in 1844 it was thought that the "cleansing of the sanctuary" meant that the Lord would return to cleans the "earth" of sin. Later, it was learned that the sanctuary that was to be cleansed was the true sanctuary in heaven.
This was the beginning of what has been called the "Investigative Judgment".
For further study (including supporting Bible texts), read: