The 1000 years starts at the first resurrection and finishes with the 2nd resurrection.
Joh 5:28 Marvel not at this: for the hour is coming, in the which all that are in the graves shall hear his voice,
Joh 5:29 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.
Rev 20:4 And I saw thrones, and they sat upon them, and judgment was given unto them: and I saw the souls of them that were beheaded for the witness of Jesus, and for the word of God, and which had not worshipped the beast, neither his image, neither had received his mark upon their foreheads, or in their hands; and they lived and reigned with Christ a thousand years.
Rev 20:5 But the rest of the dead lived not again until the thousand years were finished. This is the first resurrection.
Rev 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.
“Behold, He is coming with clouds, and every eye will see Him” (Revelation 1:7).
“The Lord Himself will descend from heaven with a shout. ... And the dead in Christ will rise first. Then we who are alive and remain shall be caught up together with them in the clouds to meet the Lord in the air” (1 Thessalonians 4:16, 17).
“There was a great earthquake, such a mighty and great earthquake as had not occurred since men were on the earth. ... And great hail from heaven fell upon men, each hailstone about the weight of a talent” (Revelation 16:18, 21).
What happens to the unsaved—living and dead—at Jesus’ second coming?
“With the breath of His lips He shall slay the wicked” (Isaiah 11:4).
“When the Lord Jesus is revealed from heaven with His mighty angels, in flaming fire taking vengeance on those who do not know God” (2 Thessalonians 1:7, 8).
“Let the wicked perish at the presence of God” (Psalm 68:2).
“The rest of the dead did not live again until the thousand years were finished” (Revelation 20:5)
The living unsaved will be slain by the very presence of Christ at the second coming. The wicked already dead when Jesus returns will remain in their graves until the end of the 1,000 years.
Jer 4:25 I beheld, and, lo, there was no man, and all the birds of the heavens were fled.
Jer 4:26 I beheld, and, lo, the fruitful place was a wilderness, and all the cities thereof were broken down at the presence of the LORD, and by his fierce anger.
Jer 25:33 And the slain of the LORD shall be at that day from one end of the earth even unto the other end of the earth: they shall not be lamented, neither gathered, nor buried; they shall be dung upon the ground.
It will be impossible for any person to repent during the 1,000 years because there will not be a person alive on the earth. The righteous will all be in heaven. All the wicked will be laying dead upon the earth. Revelation 22:11, 12 makes it clear that the case of every person is closed before Jesus returns. Those who wait to accept Christ until the 1,000 years begin will have waited too long.