Revelation 6:12-17

12 And I beheld when he had opened the sixth seal, and, lo, there was a great earthquake; and the sun became black as sackcloth of hair, and the moon became as blood;

13 And the stars of heaven fell unto the earth, even as a fig tree casteth her untimely figs, when she is shaken of a mighty wind.

14 And the heaven departed as a scroll when it is rolled together; and every mountain and island were moved out of their places.

15 And the kings of the earth, and the great men, and the rich men, and the chief captains, and the mighty men, and every bondman, and every free man, hid themselves in the dens and in the rocks of the mountains;

16 And said to the mountains and rocks, Fall on us, and hide us from the face of him that sitteth on the throne, and from the wrath of the Lamb:

17 For the great day of his wrath is come; and who shall be able to stand?

When Jesus opens the sixth seal, the great day of the wrath of God's Lamb has arrived. To announce this day there will be an earthquake so great that every island will be moved out of its place. The Sun will be darkened, the moon will become blood red and meteorites will fall from the sky.  This may be an event as earth shaking as the earth encountering a great comet or having a gravitational encounter with the heavenly city itself (the city is 1500 miles in length width and height - Revelation 21:16). This type of upheaval would account for the falling stars and for all the people hiding in caves and holes in the ground for protection. This will fulfill this passage in Joel:

Joel 2:31 The sun shall be turned into darkness, and the  moon  into blood,  before  the great and the terrible day of the LORD come.

Notice the passage says it occurs  before the terrible day of the Lord  so this is only the announcement that it is about to arrive. It is not until after the opening of the seventh seal that the wrath of the Lamb actually begins. The world will be quite aware that the time for this judgment has arrived but before judgment, God sends His prophets to allow the world to repent. The world may even miss that point when they are probably misled to believe that the two prophets of God are the biblical Beast and his false prophet.

It is my opinion that this announcement of the day of the Lord is talking about the unfulfilled week of years that Daniel says takes place after the Messiah is cut off (after Jesus was crucified). After this last week of years, the kingdom is restored to Israel. (Studies on Daniel that explain the seventy weeks of years are given in the bibliography and there are articles briefly explaining it on my website.)

Hereafter we will see God dealing with the Jews again to fulfill the final seven years.  At the mid point of these seven years, Israel has been told to flee to the mountains by Jesus and Daniel to escape the Beast. For that reason, the great final tribulation of 3 ½ years cannot happen until a future event occurs first. This event is when Satan is cast out of heaven unto the earth.

Satan appears to be cast out at the end of the sixth trumpet judgment just before he kills the two prophets of God. The prophets testify for 1260 days before they are killed.  This scenario allows 3 ½ years for the two prophets to do their Job, 3 ½ years for the Beast to fulfill the prophecy that says he will control the world for a 3 ½ year  period. Since I believe the seals occur before this final seven years, there is probably another 3 ½ year period from the opening of the first seal until the start of the testimony of the prophets.

Many teach the great day of the Lord is for only 3 ½ years (the abomination that causes desolation part certainly is) but I believe at the announcement of the day of the Lord there still remains seven years until the kingdom of God is set up. In the great day of the Lord there is first 3 ½ years for the two prophets to tell the world the truth and convince the sons of light given to Jesus to make the right choice. After that there is 3 ½ years for the counterfeit messiah possessed by Satan to lead the vessels of wrath to make the wrong choices that would bring the world to total destruction. Only the second coming of Jesus saves any flesh on the earth at all and that is done for the sake of the elect (Mark 13:20).

At the start of this seven year period Jesus seals His 144,000 Israelite witnesses. They will receive the commission to preach the truth of the gospel of the coming kingdom of Jesus as a witness to the whole world. We will read about them in the next chapter.