The day of The Lord is an expression used in the OT about God's judgment. It doesn't mean it started in OT times and continued. Instead, it means singular judgments, and there's to be a final one, which is what Apostle Paul was talking about in 2 Thess.2 with "the day of Christ". In final, it's about the time of Christ's return to end this world age. That's why in our day today, we can go back and look at those OT examples of the day of The Lord and discover Scripture within them that is meant for the final day of The Lord that is to happen with Christ's return in the near future. The OT examples serve as examples for the final one.
Jer 46:10
10 For this is the day of the Lord GOD of hosts, a day of vengeance, that He may avenge Him of His adversaries: and the sword shall devour, and it shall be satiate and made drunk with their blood: for the Lord GOD of hosts hath a sacrifice in the north country by the river Euphrates.
(KJV)
With the Jeremiah example, it was about God using Babylon to bring a sacrifice upon Egypt, conquering Egypt to the border of Euphrates. But in Rev.16 it's about God opening up the symbolic border of Euphrates so the kings of the east might be prepared for the final battle, a battle that Christ and His angels will fight in final.
2 Pet 3:10-12
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.
11 Seeing then that all these things shall be dissolved, what manner of persons ought ye to be in all holy conversation and godliness,
12 Looking for and hasting unto the coming of the day of God, wherein the heavens being on fire shall be dissolved, and the elements shall melt with fervent heat?
(KJV)
Jer 46:10
10 For this is the day of the Lord GOD of hosts, a day of vengeance, that He may avenge Him of His adversaries: and the sword shall devour, and it shall be satiate and made drunk with their blood: for the Lord GOD of hosts hath a sacrifice in the north country by the river Euphrates.
(KJV)
With the Jeremiah example, it was about God using Babylon to bring a sacrifice upon Egypt, conquering Egypt to the border of Euphrates. But in Rev.16 it's about God opening up the symbolic border of Euphrates so the kings of the east might be prepared for the final battle, a battle that Christ and His angels will fight in final.
2 Pet 3:10-12
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.
11 Seeing then that all these things shall be dissolved, what manner of persons ought ye to be in all holy conversation and godliness,
12 Looking for and hasting unto the coming of the day of God, wherein the heavens being on fire shall be dissolved, and the elements shall melt with fervent heat?
(KJV)