Different views place the time of the tribulation at different points in relation to the Millennium (which means 1,000 years). In the book of Revelation, the apostle John describes the vast multitude of people who come out of this time of testing and trial.
These who are clothed with white robes, who are they, and where did they come from? . . . They are the ones who have come out of the great tribulation; and they have washed their robes, and have made their robes white in the blood of the Lamb (Revelation 7).
To some teachers of prophecy, the meaning of the above verses is that they are referencing a specific period called Tribulation. Based on a comparison of relevant prophetic scriptures, many people feel this time of trial will last seven years.
While some churches and groups reference the entire seven year period by the phrase "great tribulation," others refer only to the last half or three and one half years as being Tribulation. This is done because it is believed the last half of the seven-year period will bring far worse suffering and trial than the first half.
Additionally, many try to figure out WHEN this time of global tribulation, suffering and trials will occur. What is known as Jesus' Olivet prophecies, recorded in Matthew 24, Mark 13, and Luke 21, are used to help determine exactly when the end will come and the Second Coming occurs.
When his disciples asked Jesus what would be the signs of his coming to earth during the end time he stated several events. He first warned them to watch so that they would not be fooled. Jesus then stated the following.
And you shall hear of wars and rumors of wars. See that you do not let these things disturb you. For it is necessary that all these things take place, but the end is not yet. For nation shall rise against nation, and kingdom against kingdom; and there shall be famines and pestilences and earthquakes in different places (Matthew 24:6 - 7).
During the great Tribulation, the devil will deceive the entire world into worshipping him in the form of the end time Antichrist (see Revelation 13:1, 4). He will eventually gather all the armies of the world to fight against a returning Jesus Christ (Revelation 16:12, 14).