What is supposed to happen on that day?
Malachi 4:5 Behold, I will send you Elijah the prophet before the coming of the great and dreadful day of the LORD:
1 Thessalonians 5:2 For yourselves know perfectly that the day of the Lord so cometh as a thief in the night.
Matthew 25:13 Watch therefore, for ye know neither the day nor the hour wherein the Son of man cometh.
The day of the Lord is the time of Christ’s return to judge the world and establish the kingdom of God
Acts 17:31 Because he hath appointed a day, in the which he will judge the world in righteousness by that man whom he hath ordained; whereof he hath given assurance unto all men, in that he hath raised him from the dead.
When he returns he will raise the dead and judge them.
John 5:28 Marvel not at this: for the hour is coming, in the which all that are in the graves shall hear his voice, 29 And shall come forth; they that have done good, unto the resurrection of life; and they that have done evil, unto the resurrection of damnation.
He will restore the kingdom of Israel and rule the earth from Jerusalem.
Luke 1:32 He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto him the throne of his father David: 33 And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end.
Psalms 2:6 Yet have I set my king upon my holy hill of Zion. 7 I will declare the decree: the LORD hath said unto me, Thou art my Son; this day have I begotten thee. 8 Ask of me, and I shall give thee the heathen for thine inheritance, and the uttermost parts of the earth for thy possession. 9 Thou shalt break them with a rod of iron; thou shalt dash them in pieces like a potter’s vessel.
Isaiah 2:2 And it shall come to pass in the last days, that the mountain of the LORD’S house shall be established in the top of the mountains, and shall be exalted above the hills; and all nations shall flow unto it. 3 And many people shall go and say, Come ye, and let us go up to the mountain of the LORD, to the house of the God of Jacob; and he will teach us of his ways, and we will walk in his paths: for out of Zion shall go forth the law, and the word of the LORD from Jerusalem. 4 And he shall judge among the nations, and shall rebuke many people: and they shall beat their swords into plowshares, and their spears into pruninghooks: nation shall not lift up sword against nation, neither shall they learn war any more.
Christ will return at a time when Israel is being overrun by a northern invader as described in Ezekiel 38 and 39 and Zechariah 12 to 14.
Christ will save and restore Israel and rule over the mortal population for 1000 years.
This will be the fulfilment of Daniel’s prophecy:
Daniel 2:44 And in the days of these kings shall the God of heaven set up a kingdom, which shall never be destroyed: and the kingdom shall not be left to other people, but it shall break in pieces and consume all these kingdoms, and it shall stand for ever.
This overturning of the kingdoms of men is described by Peter.
2 Peter 3:10 But the day of the Lord will come as a thief in the night; in the which the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also and the works that are therein shall be burned up.
Peter is to be understood figuratively not literally. The heavens refers to the political heavens – governments etc. The earth and the elements refers to the social structures, laws and cultures of those who are governed.
All will be taught the truth and ruled by the righteous laws given by Christ.
Those who are accepted at the resurrection will reign with him.
Revelation 20:6 Blessed and holy is he that hath part in the first resurrection: on such the second death hath no power, but they shall be priests of God and of Christ, and shall reign with him a thousand years.
The apostles will be appointed as judges over the twelve tribes of Israel.
Matthew 19:28 And Jesus said unto them, Verily I say unto you, That ye which have followed me, in the regeneration when the Son of man shall sit in the throne of his glory, ye also shall sit upon twelve thrones, judging the twelve tribes of Israel.
At the end of the millennial reign there will be a second resurrection and final judgment. Sin and death will be eradicated and all who live on will be given immortality. God will live in and with his people forever.
This will be the fulfilment of God’s purpose.
Numbers 14:21 But as truly as I live, all the earth shall be filled with the glory of the LORD.
Paul explains what is going to happen:
1 Corinthians 15:22 For as in Adam all die, even so in Christ shall all be made alive. 23 But every man in his own order: Christ the firstfruits; afterward they that are Christ’s at his coming. 24 Then cometh the end, when he shall have delivered up the kingdom to God, even the Father; when he shall have put down all rule and all authority and power. 25 For he must reign, till he hath put all enemies under his feet. 26 The last enemy that shall be destroyed is death. 27 For he hath put all things under his feet. But when he saith all things are put under him, it is manifest that he is excepted, which did put all things under him. 28 And when all things shall be subdued unto him, then shall the Son also himself be subject unto him that put all things under him, that God may be all in all.
This time is described in Revelation 21
Revelation 21:3 And I heard a great voice out of heaven saying, Behold, the tabernacle of God is with men, and he will dwell with them, and they shall be his people, and God himself shall be with them, and be their God. 4 And God shall wipe away all tears from their eyes; and there shall be no more death, neither sorrow, nor crying, neither shall there be any more pain: for the former things are passed away.
I hope you find this helpful.