1Now, brothers, concerning the coming of our Lord Jesus Christ and our gathering together to him, we ask you 2not to be quickly shaken in your mind or troubled, either by spirit or by word or by letter as if from us, saying that the day of Christ has already come. 3Let no one deceive you in any way. For it will not be unless the rebellion comes first, and the man of sin is revealed, the son of destruction. 4He opposes and exalts himself against all that is called God or that is worshiped, so that he sits as God in the temple of God, setting himself up as God. 5Don’t you remember that when I was still with you, I told you these things? 6Now you know what is restraining him, to the end that he may be revealed in his own season. 7For the mystery of lawlessness already works. Only there is one who restrains now, until he is taken out of the way. 8Then the lawless one will be revealed, whom the Lord will kill with the breath of his mouth and destroy by the manifestation of his coming; 9even he whose coming is according to the working of Satan with all power and signs and lying wonders, 10and with all deception of wickedness for those who are being lost, because they didn’t receive the love of the truth, that they might be saved. 11Because of this, God sends them a powerful delusion, that they should believe a lie, 12that they all might be judged who didn’t believe the truth, but had pleasure in unrighteousness.
13But we are bound to always give thanks to God for you, brothers loved by the Lord, because God chose you from the beginning for salvation through sanctification of the Spirit and belief in the truth, 14to which he called you through our Good News, for the obtaining of the glory of our Lord Jesus Christ. 15So then, brothers, stand firm and hold the traditions which you were taught by us, whether by word or by letter.
16Now our Lord Jesus Christ himself, and God our Father, who loved us and gave us eternal comfort and good hope through grace, 17comfort your hearts and establish you in every good work and word.
To the minds of the Hebrew, the day of the Lord was the future day when God would intervene in human history to vindicate Himself and the people of Israel. This concept seems to have appeared during the Conquest when God judged the Canaanites on behalf of His people Israel.
When Amos incorporated the concept of the day of the Lord in his preaching, however, his view was shocking to his people because Amos' understanding of this day was in total contrast to the popular notion of the 8th century B.C. Amos proclaimed that Israel would lament rather than rejoice on that day (Amos 5:16-17) for God was to judge Israel (5:18-20) and the world. On that day, God was going to reject Israel's meaningless religious rituals (5:21-27) and it was to be a day of darkness and woe (6:1-6). The moral corruption of Israel would ultimately usher in Israel's destruction on the day.
Expressing the concept in variant forms such as "by that day" and "on that day," the prophets frequently used this concept in their proclamation of judgment and hope. Zephaniah used it seven times, Isaiah six times, and Joel five times. These are the characteristics of the day of the Lord: 1) It will be a day of judgment (Joel 3:19-21; Amos 5:18) on the enemies of God including Israel and a day of vindication for the faithful; 2) The day of the Lord will be experienced everywhere (Isa 2:12; Zeph 1, 2; Amos 5:18-20); 3) The day of the Lord is near--not far out in the future (Isa 13:6); 4) There will be transformation in nature (Joel 3:18; Amos 9:13), in human relationships (Mic 4:3), and in the relationship between human beings and animals (Isa 11:6-8) which will result in the emergence of a better world; 5) On that day, Jerusalem will be exalted (Zech 14:16); 6) It will be accompanied by cosmic phenomena (Joel 2:28-32; Isa 13:10; 34:4; Ezek 32:7-8); 7) There will be a Davidic ruler (Isa 11:10); 8) There will be a re-gathering of God’s people (Isa 11:11). The New Testament equivalent for this is the second coming of Jesus Christ (2 Thess 2:1) and the day of the Lord Paul mentions in 2:2.