If you knew what it will be that the Lord will use on His terrible Day of vengeance and wrath, then you would understand how Joel 2:31 does relate to the Sixth Seal event.
Isaiah 30:26 and Psalms 50:1-4 both refer to the sun, a yet to occur; huge explosion of the suns surface; a Coronal Mass Ejection of unprecedented magnitude.
We will see the initial flash of this explosion, as we do see many small CME's, recorded by NASA's SOHO satellites.
Then the sun will darken, as the earth directed mass obscures it, the Moon will glow blood red from the reflected flash and more so later, from the thermolumiscent reaction as it is struck by the superheated Hydrogen plasma ejecta.
It's rather obvious that Joel 2:31 and Revelation 6:12-17 are the same event; The Day of the Lord, when He destroys His enemies. As both Prophesies specifically state.
The last week is a seven year time period.
The middle of the week is 3 and a half years into the seven year period.
It says in Joel 2:31 that these signs are BEFORE the “
Day of the Lord”
(a period of time of God’s wrath) takes place. So the sixth seal that describes these same signs must happen right before the “
Day of the Lord” commences. The sixth seal only works logically on the timeline sometime not too long after the Middle of the Tribulation. Each of the seals plays off one another like a chain reaction or domino effect.
The Middle of the Tribulation:
(Also Known As the "Middle of Daniel's 70th Week"):
The Beast Takes Jerusalem & The Temple Sacrifices Are Stopped:
(And the Gentile Nations Will Trod Down Upon the City for 42 Months):
• Daniel 11:31a NIV
His armed forces will rise up to desecrate the temple fortress and will abolish the daily sacrifice.
• Daniel 12:11a NLT and Daniel 12:11b NLT
From the time the daily sacrifice is stopped... there will be 1,290 days.
• Revelation 11:2 NLT
The nations ... will trample the holy city for 42 months.
Midpoint Day of the Tribulation:
(Continuation of the "Middle of Daniel's 70th Week"):
The Beast & the False Prophet Set Up the Abomination of Desolations in the Jewish Temple:
• Revelation 13:14-15
They... make an image to the beast, that had the wound by a sword, and did live. And he had power to give life unto the image of the beast, that the image of the beast should both speak, and cause that as many as would not worship the image of the beast should be killed.
• Matthew 24:15-22
When ye therefore shall see the abomination of desolation, spoken of by Daniel the prophet, stand in the holy place, (whoso readeth, let him understand): Then let them which be in Judaea flee into the mountains: Let him which is on the housetop not come down to take any thing out of his house: Neither let him which is in the field return back to take his clothes. And woe unto them that are with child, and to them that give suck in those days! But pray ye that your flight be not in the winter, neither on the sabbath day: For then shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be. And except those days should be shortened, there should no flesh be saved: but for the elect's sake those days shall be shortened.
• Mark 13:14-20
But when ye shall see the abomination of desolation, spoken of by Daniel the prophet, standing where it ought not, (let him that readeth understand,) then let them that be in Judaea flee to the mountains: And let him that is on the housetop not go down into the house, neither enter therein, to take any thing out of his house: And let him that is in the field not turn back again for to take up his garment. But woe to them that are with child, and to them that give suck in those days! And pray ye that your flight be not in the winter. For in those days shall be affliction, such as was not from the beginning of the creation which God created unto this time, neither shall be. And except that the Lord had shortened those days, no flesh should be saved: but for the elect's sake, whom he hath chosen, he hath shortened the days.
• Daniel 9:24-27
Seventy weeks are determined upon thy people and upon thy holy city, to finish the transgression, and to make an end of sins, and to make reconciliation for iniquity, and to bring in everlasting righteousness, and to seal up the vision and prophecy, and to anoint the most Holy. Know therefore and understand, that from the going forth of the commandment to restore and to build Jerusalem unto the Messiah the Prince shall be seven weeks, and threescore and two weeks: the street shall be built again, and the wall, even in troublous times. And after threescore and two weeks shall Messiah be cut off, but not for himself: and the people of the prince that shall come shall destroy the city and the sanctuary; and the end thereof shall be with a flood, and unto the end of the war desolations are determined. And he shall confirm the covenant with many for one week: and in the midst of the week he shall cause the sacrifice and the oblation to cease, and for the overspreading of abominations he shall make it desolate, even until the consummation, and that determined shall be poured upon the desolate.
A Select Group of Jews Flee to the Mountains:
(Which can also be referred to as "the wilderness" within the Scriptures)
• Revelation 12:6
And the woman fled into the wilderness, where she has a place prepared by God, that they should feed her there a thousand two hundred and three score days.
• Revelation 12:14-16
And to the woman were given two wings of a great eagle, that she might fly into the wilderness, into her place, where she is nourished for a time, and times, and half a time, from the face of the serpent. And the serpent cast out of his mouth water as a flood after the woman, that he might cause her to be carried away by the flood. And the earth helped the woman, and the earth opened its mouth, and swallowed up the flood which the dragon cast out of his mouth.
What we read in the Olivet discourse is the middle of the Tribulation and not the End. Logically, Revelation 19 (the Second Coming of our Lord) does not sound like the same event as the Olivet discourse. Jesus is coming for His remaining saints in the Olivet Discourse (Middle of the Tribulation) whereas Revelation 19 describes His saints following Him (down into battle). Any trained detective or impartial observer looking at the evidence can tell this. Only those who have a persecution complex and or who cannot do proper detective work are going to conclude the wrong thing. They see a Rapture at the end happening right before Jesus’ return or no Rapture at all taking place. This does not make any sense in light of what we read in Scripture. I think also some believers have a hard time receiving good gifts from God if they are faithful to Him. Part of Hebrews 11:6 basically talks about how God is rewarder to those who diligently seek Him. Meaning, we must believe that. We must believe God is a rewarder to those who diligently seek Him.
To say that the Olivet Discourse is the last day of the Tribulation contradicts the 42 months that must happen afterwards and Daniel’s prophecy of timeline. There will still be months where the Gentiles trodden down the temple after the Lord returns? That is why the post view end times view does not make any logical sense. One is ignoring the framework of the time line (Which throws other verses off in not making any sense).
Jesus even warns us not to say that the Lord delays His coming (Which is what I believe the Post End Times position promotes). Jesus taught that His coming was imminent and can happen at any moment whereby we would not know the day or the hour. We could be caught off guard (thinking He will arrive later), and so that is why we must be faithful to the Lord Jesus in this life and look for His coming at any moment (As Scripture says). If we are not faithful to Jesus in this life and He comes, or we die, we are not going to enter God’s Kingdom (Which same rule applies even to me as everyone else). 
