The Great Tribulation

(Mt 24:21, 22) For then there will be great tribulation such as has not occurred since the world's beginning until now, no, nor will occur again. In fact, unless those days were cut short, no flesh would be saved; but on account of the chosen ones those days will be cut short.

When it begins

Daniel pins down the start. The king of the north comes to his end with no one to help him (Daniel 11:45), and right then:

(Da 12:1) And during that time Michael will stand up, the great prince who is standing in behalf of the sons of your people. And there will certainly occur a time of distress such as has not been made to occur since there came to be a nation until that time. And during that time your people will escape, every one who is found written down in the book.

But the great tribulation isn't the pangs of distress. The pangs are the birth pains that lead up to it, and the tribulation is the birth.

What happens inside it

The studies on this site read the sequence like this. The seventh head of the beast takes its death-stroke and is healed, and the eighth king ascends from the abyss (Revelation 13:3; 17:8). The two witnesses prophesy for 1,260 days, and their testimony exposes Babylon the Great. Then Jehovah puts it into the hearts of the ten kings and the beast to devastate the harlot (Revelation 17:16, 17), which clears away the fog so that every person faces a plain choice.

After that the image of the beast is set up and given breath, and the mark is enforced on pain of death (Revelation 13:15-17). The beast kills the witnesses, and after three and a half days they are raised (Revelation 11:7-12). Then the sign of the Son of man appears (Matthew 24:30), and the kings of the earth are gathered to Armageddon (Revelation 16:14-16), where the eighth king and its image go off into destruction.

Who comes through it

(Re 7:14) These are the ones that come out of the great tribulation, and they have washed their robes and made them white in the blood of the Lamb.

The days are cut short on account of the chosen ones. The great crowd that comes out of the tribulation has its own entry. What matters for anyone reading now is that the tribulation runs in a fixed order, and knowing that order lets you see where the world stands and act before the choice is forced on you.