The day of the Lord is a BIG day. It is a specific point of time at the end of human history as we know it. It is also ONE day–not a series of days or period of time. Many things happen all in One day at the end of the great tribulation…when Jesus returns to earth at His second coming (Heb. 9:28) on the LAST day of human history.
This is the will of the Father who sent Me, that of all He has given Me I should lose nothing, but should raise it up at the last day. And this is the will of Him who sent Me, that everyone who sees the Son and believes in Him may have everlasting life; and I will raise him up at the last day. – John 6:39,40, NKJV
How do we know that the day of the Lord is the last day at the second coming of Christ? God revealed as much to the prophet Zechariah:
And in that day His feet will stand on the Mount of Olives, which faces Jerusalem on the east. and the Mount of Olives shall be split in two, …Thus the Lord my God will come, and all the saints with You. It shall come to pass in that day that there will be no light; the lights will diminish. It shall be one day which is known to the Lord—neither day nor night. but at evening time it shall happen that it will be light. And in that day it shall be that living waters shall flow from Jerusalem, half of them toward the eastern sea and half of them toward the western sea; In both summer and winter it shall occur. And the Lord shall be King over all the earth. In that day it shall be—“The Lord is one,” And His name one. – Zechariah 14:5-9, NKJV
What Zechariah is saying is that Jesus (who is God) is returning to the Mount of Olives at His second coming (Acts 1:9-12). He is bringing the saints with Him because they have just been resurrected (and the living saints gathered/raptured) because it is also the last day. In ONE day, Jesus will return to the Mount of Olives with His saints while it is dark (because the sun and moon do not give their light), and when it is evening, after everything else happens, it will be light.
Jesus explained the darkness and His second coming to His apostles when they asked Him about the end days:
29 “Immediately after the tribulation of those days the sun will be darkened, and the moon will not give its light; the stars will fall from heaven, and the powers of the heavens will be shaken. 30 Then the sign of the Son of Man will appear in heaven, and then all the tribes of the earth will mourn, and they will see the Son of Man coming on the clouds of heaven with power and great glory. 31 And He will send His angels with a great sound of a trumpet, and they will gather together His elect from the four winds, from one end of heaven to the other. – Matthew 24:29-31, NKJV
This is referring to the last day of human history after the tribulation has ended. It is the day of the Lord and His return. Darkness will fill the atmosphere and then Jesus will return to earth, bringing the saints with Him as they are resurrected and raptured/gathered all together (I Thes. 4:14-17).
How do we know Jesus was equating His second coming with "the day of the Lord"? Other scripture uses that term or similar, speaking of the darkness, earthly changes, and the wrath of God that is also going to occur on this last day of human history:
The sun shall be turned into darkness, and the moon into blood, before the coming of the great and awesome day of the Lord. – Joel 2:31, NKJV
But who can endure the day of His coming? And who can stand when He appears? For He is like a refiner’s fire And like launderers’ soap. – Mal. 3:2, NKJV
12 I looked when He opened the sixth seal, and behold, there was a great earthquake; and the sun became black as sackcloth of hair, and the moon became like blood. 13 And the stars of heaven fell to the earth, as a fig tree drops its late figs when it is shaken by a mighty wind. 14 Then the sky receded as a scroll when it is rolled up, and every mountain and island was moved out of its place. 15 And the kings of the earth, the great men, the rich men, the commanders, the mighty men, every slave and every free man, hid themselves in the caves 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 who sits on the throne and from the wrath of the Lamb! 17 For the great day of His wrath has come, and who is able to stand?” – Rev. 6:12-17
10 But the day of the Lord will come as a thief in the night, in which the heavens will pass away with a great noise, and the elements will melt with fervent heat; both the earth and the works that are in it will be burned up. 11 Therefore, since all these things will be dissolved, what manner of persons ought you to be in holy conduct and godliness, 12 looking for and hastening the coming of the day of God, because of which the heavens will be dissolved, being on fire, and the elements will melt with fervent heat? 13 Nevertheless we, according to His promise, look for new heavens and a new earth in which righteousness dwells. – II Peter 3:9-13, NKJV
14 For they are spirits of demons, performing signs, which go out to the kings of the earth and of the whole world, to gather them to the battle of that great day of God Almighty. 15 “Behold, I am coming as a thief. Blessed is he who watches, and keeps his garments, lest he walk naked and they see his shame.” 16 And they gathered them together to the place called in Hebrew, Armageddon. – Rev. 16:14-16, NKJV
A lot is going on this day–good things for believers who have now obtained glorified bodies and will reign with Christ for 1000 years (Phil. 3:21; Rev. 20:4). But unbelievers are not going to have a good day. The day of the Lord is also the day of the Lord's vengeance. Many unbelievers who are still alive when Christ returns will die at His return.
7 and to give you who are troubled rest with us when the Lord Jesus is revealed from heaven with His mighty angels, 8 in flaming fire taking vengeance on those who do not know God, and on those who do not obey the gospel of our Lord Jesus Christ. 9 These shall be punished with everlasting destruction from the presence of the Lord and from the glory of His power, 10 when He comes, in that Day, to be glorified in His saints and to be admired among all those who believe, because our testimony among you was believed. – II Thessalonians 1:7-10, NKJV
The day of the Lord is the day of God's wrath. Scripture refers to this day in various ways:
in the day of His fierce anger (Isaiah 13:13).
For it is the day of the Lord’s vengeance (Isaiah 34:8).
in the day of doom (Jer. 51:2).
their gold will not be able to deliver them In the day of the wrath of the Lord (Ez. 7:16).
A day of devastation and desolation (Zeph. 1:15).
The battle of Armageddon also happens this day (mentioned above, Rev. 16:14-16) which involves tens of thousands of soldiers who will die in battle. Many soldiers are marching against Jerusalem, which brings us to another important part of the day of the Lord…survivors.
3 On that day I will make Jerusalem a heavy stone for all the peoples; all who try to lift it will injure themselves severely when all the nations of the earth gather against her. 4 On that day”—this is the Lord’s declaration—“I will strike every horse with panic and its rider with madness. I will keep a watchful eye on the house of Judah but strike all the horses of the nations with blindness. 5 Then each of the leaders of Judah will think to himself: The residents of Jerusalem are my strength through the Lord of Hosts, their God. 6 On that day I will make the leaders of Judah like a firepot in a woodpile, like a flaming torch among sheaves; they will consume all the peoples around them on the right and the left, while Jerusalem continues to be inhabited on its site, in Jerusalem. 7 The Lord will save the tents of Judah first, so that the glory of David’s house and the glory of Jerusalem’s residents may not be greater than that of Judah. 8 On that day the Lord will defend the inhabitants of Jerusalem, so that on that day the one who is weakest among them will be like David on that day, and the house of David will be like God, like the Angel of the Lord, before them. 9 On that day I will set out to destroy all the nations that come against Jerusalem. – Zech. 12:3-9, HCSB
On the day of Christ's return, He is going to defend Jerusalem as the armies of nations move against them. He will be defending living Jews and will "destroy all the nations that come against Jerusalem." What is key is that living Jews (and other Israelites spread throughout the world) realize that Jesus is their Messiah for the first time when He returns in the clouds (Rev. 1:7). However, it is too late for them to get raptured/gathered with the rest of the living believers who are joining Christ in the resurrection.
10 “And I will pour on the house of David and on the inhabitants of Jerusalem the Spirit of grace and supplication; then they will look on Me whom they pierced. Yes, they will mourn for Him as one mourns for his only son, and grieve for Him as one grieves for a firstborn. 11 In that day there shall be a great mourning in Jerusalem, like the mourning at Hadad Rimmon in the plain of Megiddo. 12 And the land shall mourn, every family by itself: the family of the house of David by itself, and their wives by themselves; the family of the house of Nathan by itself, and their wives by themselves; 13 the family of the house of Levi by itself, and their wives by themselves; the family of Shimei by itself, and their wives by themselves; 14 all the families that remain, every family by itself, and their wives by themselves. – Zechariah 12:10-14, NKJV
So on the day of the Lord, unbelieving Israel (those who did not receive the mark of the beast) will finally recognize Jesus as their Messiah and will remain as survivors in the millennial kingdom of Christ on earth. And that's a whole other story.