The Day of the Lord.
According to Mal 4:5-6 Elijah is one of the two witnesses who will be sent back before the great and terrible day of the Lord. From Rev 11 we know that the two witnesses will appear during the great tribulation period. From this we can conclude that the day of the Lord occurs after the great tribulation period.
Mal 4:5-6 "Behold, I am going to send you Elijah the prophet before the coming of the great and terrible day of the LORD. 6 "And he will restore the hearts of the fathers to their children, and the hearts of the children to their fathers, lest I come and smite the land with a curse.”
Acts 2:20 tells us the day of the Lord occurs after the great tribulation period, which is metaphorically described in the phrase “the sun being turned into darkness and the moon into blood”.
Acts 2:20 'THE SUN SHALL BE TURNED INTO DARKNESS, AND THE MOON INTO BLOOD, BEFORE THE GREAT AND GLORIOUS DAY OF THE LORD SHALL COME.
Matt. 24: 15-21 tells us the great tribulation period, begins when the abomination of desolation is set up.
Matt. 24:15 “Therefore when you see the ABOMINATION OF DESOLATION which was spoken of through Daniel the prophet, standing in the holy place ( let the reader understand), 16 then let those who are in Judea flee to the mountains; 17 let him who is on the housetop not go down to get the things out that are in his house; 18 and let him who is in the field not turn back to get his cloak. 19 “But woe to those who are with child and to those who nurse babes in those days! 20 “But pray that your flight may not be in the winter, or on a Sabbath; 21 for then there will be a great tribulation, such as has not occurred since the beginning of the world until now, nor ever shall.
2Th 2:2 tells us the coming of our Lord and our gathering together to him or resurrection found in 2Th 2:1 marks the beginning of the day of the Lord.
2 Th 2:1-2 Now we request you, brethren, with regard to the coming of our Lord Jesus Christ, and our gathering together to Him, 2 that you may not be quickly shaken from your composure or be disturbed either by a spirit or a message or a letter as if from us, to the effect that the day of the Lord has come.
According to 2Pet 3:10 the day of the Lord continues until the end of the age when the heavens and earth are destroyed and the judgment of the dead occurs.
2 Pet 3:10 But the day of the Lord will come like a thief, in which the heavens will pass away with a roar and the elements will be destroyed with intense heat, and the earth and its works will be burned up.
Zec 14:6-21 provides a glimpse into what the day of the Lord will be like during the 1000 year reign.
Zech. 14:6 And it will come about in that day that there will be no light; the luminaries will dwindle. 7 For it will be a unique day which is known to the LORD, neither day nor night, but it will come about that at evening time there will be light. 8 And it will come about in that day that living waters will flow out of Jerusalem, half of them toward the eastern sea and the other half toward the western sea; it will be in summer as well as in winter.
Zech. 14:9 And the LORD will be king over all the earth; in that day the LORD will be the only one, and His name the only one. 10 All the land will be changed into a plain from Geba to Rimmon south of Jerusalem; but Jerusalem will rise and remain on its site from Benjamin’s Gate as far as the place of the First Gate to the Corner Gate, and from the Tower of Hananel to the king’s wine presses. 11 And people will live in it, and there will be no more curse, for Jerusalem will dwell in security.
Zech. 14:12 Now this will be the plague with which the LORD will strike all the peoples who have gone to war against Jerusalem; their flesh will rot while they stand on their feet, and their eyes will rot in their sockets, and their tongue will rot in their mouth. 13 And it will come about in that day that a great panic from the LORD will fall on them; and they will seize one another’s hand, and the hand of one will be lifted against the hand of another. 14 And Judah also will fight at Jerusalem; and the wealth of all the surrounding nations will be gathered, gold and silver and garments in great abundance. 15 So also like this plague, will be the plague on the horse, the mule, the camel, the donkey, and all the cattle that will be in those camps.
Zech. 14:16 Then it will come about that any who are left of all the nations that went against Jerusalem will go up from year to year to worship the King, the LORD of hosts, and to celebrate the Feast of Booths. 17 And it will be that whichever of the families of the earth does not go up to Jerusalem to worship the King, the LORD of hosts, there will be no rain on them. 18 And if the family of Egypt does not go up or enter, then no rain will fall on them; it will be the plague with which the LORD smites the nations who do not go up to celebrate the Feast of Booths. 19 This will be the punishment of Egypt, and the punishment of all the nations who do not go up to celebrate the Feast of Booths. 20 In that day there will be inscribed on the bells of the horses, “ HOLY TO THE LORD.” And the cooking pots in the LORD’S house will be like the bowls before the altar. 21 And every cooking pot in Jerusalem and in Judah will be holy to the LORD of hosts; and all who sacrifice will come and take of them and boil in them. And there will no longer be a Canaanite in the house of the LORD of hosts in that day.
If I was going to sum up the Day of the Lord I would say it is when Jesus intervenes in the affairs of mankind and includes the resurrection, Armageddon, the disposal of the Antichrist, False Prophet and his armies, Jesus’s 1000 years reign, the destruction of the heavens and earth, the judgment of the dead, the sentencing of the unsaved to the Lake of Fire and the new heaven and the new earth.