In 1 Thess 5:2 Paul speaks of the Day of the Lord, concerning which no-one knows the time. Later in 2 Thess 2:2 he speaks of the Day of the Lord and then says that it cannot be at hand since the great Apostasy and the Man of Lawlessness have not yet appeared. Let us focus on the “Day” terms.
Some students assume that a literal exegesis must lead to the conclusion that, for example, the Day of Christ is quite different from the Day of the Lord. In fact, a study of these phrases reveals that Paul uses such terms as Day of the Lord, Day of Christ, Day of the Lord Jesus Christ interchangeably, without intending to lend fine shades of meaning to each one. His readers did not need a decoder ring in order to “rightly divide” his letters, since all the “Day” references as variations of the “Day of Yahweh” from the Old Testament.
What is momentous from the standpoint of Paul’s christology is that the Day of Yahweh predictions find their fulfillment in the coming of the Lord Jesus; all of the “Day” references are therefore indirect but sturdy affirmations of Christ’s deity. Compare for example, “That day is the day of the Lord Yahweh of hosts, a day of retribution, to gain vindication from his foes” (Jer 46:10 NRSV, paraphrased by author) with 2 Thess 1:8b, 10a, “when the Lord Jesus is revealed from heaven with his mighty angels in flaming fire, inflicting vengeance…on that day.” Some scholars have suggested that when early gentile Christians used the Septuagint they mistakenly applied Yahweh passages to the Lord Jesus, since the LXX used “Lord” (kurios) to translate Yahweh and Adonai. But the apostle Paul cannot be confused on that score. He has a firm grasp both on the Hebrew text and the Septuagint; when he refers to Yahweh passages as predictions of the Lord Jesus, he is consciously implying that Jesus is Yahweh.
See especially the fine study of this phenomenon by Gordon Fee: Pauline Christology (2007); The First and Second Letters to the Thessalonians (2009).
The data from Paul’s epistles: Day of the Lord Jesus Christ (1 Cor 1:8); the Day of Jesus Christ (Phil 1:6); the Day of the Lord Jesus (1 Cor 5:5; 2 Cor 1:14); or simply the “Day” (1 Cor 3:13; 2 Thess 1:10; 2:3; 2 Tim 1:12; 1:18; 4:8); Day of Christ (Phil 1:10; 2:16). Day of Christ is supported in 2 Thess 2:2 by the second corrector of the manuscript D, the Majority Text and the Textus Receptus (thus the KJV), but “Day of the Lord” has much better manuscript support.
The “Day of the Lord” in Paul’s Letters: what does it say about Jesus? [Studies in Thessalonians], by Gary Shogren, PhD in New Testament Exegesis, Seminario ESEPA, San José, Costa Rica