Answer: The phrase “day of the Lord” usually identifies events that take place at the end of history (Isaiah 7:18-25) and is often closely associated with the phrase “that day.”
One key to understanding these phrases is to note that they always identify a span of time during which God personally intervenes in history, directly or indirectly, to accomplish some specific aspect His plan.
Most people associate “the day of the Lord” with a period of time or a special day that will occur at the end of time when God’s will and purpose for His world and for mankind will be fulfilled. Some scholars believe that “the day of the Lord” will be a longer period of time, rather than a single day—a period of time when Christ will reign throughout the world before He cleanses heaven and earth in preparation for the eternal state of all mankind. Yet other scholars believe the day of the Lord will be an instantaneous event when Christ returns to earth to redeem His faithful believers and send unbelievers to eternal damnation.
The phrase “the day of the Lord” is used nineteen times in the Old Testament (Isaiah 2:12; 13:6, 9; Ezekiel 13:5, 30:3; Joel 1:15, 2:1,11,31; 3:14; Amos 5:18,20; Obadiah 15; Zephaniah 1:7,14; Zechariah 14:1; Malachi. 4:5) and four times in the New Testament (Acts 2:20; 2 Thessalonians 2:2; 2 Peter 3:10). It is also alluded to in other passages (Revelation 6:17; 16:14).
The Old Testament passages dealing with the day of the Lord often convey a sense of imminence, nearness, and expectation: “Wail, for the day of the Lord is near!” (Isaiah 13:6); “For the day is near, even the day of the Lord is near” (Ezekiel 30:3); “For the day of the Lord is near” (Joel 1:15); “Let all the inhabitants of the land tremble, for the day of the Lord is coming; surely it is near” (Joel 2:1); “Multitudes, multitudes in the valley of decision! For the day of the Lord is near in the valley of decision” (Joel 3:14); “For the day of the Lord draws near on all the nations” (Obadiah 15); “Be silent before the Lord God! For the day of the Lord is near” (Zephaniah 1:7); “Near is the great day of the Lord, near and coming very quickly” (Zephaniah1:14). This is because the Old Testament “day of the Lord” passages often speak of both a near and a far fulfillment, as does much of Old Testament prophecy. There are times in the Old Testament that “the day of the Lord” is used to describe historical judgments that have already been fulfilled in at least some sense (Isaiah 13:6-22; Ezekiel 30:2-19; Joel 1:15; 3:14; Amos 5:18-20; Zephaniah 1:14-18), while at other times it refers to divine judgments that will take place towards the end of the age (Joel 2:30-32; Zechariah 14:1; Malachi 4:1, 5).
The New Testament calls it a day of “wrath,” a day of “visitation,” and the “great day of God Almighty” (Revelation 16:4) and refers to a still-future fulfillment when God’s wrath is poured out on unbelieving Israel (Isaiah 22; Jeremiah 30:1-17; Joel 1-2; Amos 5; Zephaniah 1) and on the unbelieving world (Ezekiel 38–39; Zechariah 14). The Scriptures indicate that “the day of the Lord” will come quickly, like a thief in the night (Zephaniah 1:14-15; 2 Thessalonians 2:2), and therefore we as Christians must be watchful and ready for the coming of Christ at any moment.
Besides being a time of judgment, it will also be a time of salvation as God will deliver the remnant of Israel, fulfilling His promise that “all of Israel will be saved” (Romans 11:26), forgiving their sins and restoring His chosen people to the land He promised to Abraham (Isaiah 10:27; Jeremiah 30:19-24, 40; Micah 4; Zechariah 13). The final outcome of the day of the Lord will be that “the arrogance of man will be brought low and the pride of men humbled; the Lord alone will be exalted in that day” (Isaiah 2:17). The ultimate or final fulfillment of the prophecies concerning “the day of the Lord” will come at the end of history when with wondrous power God will punish evil and fulfill all His promises.
Original Source: gotquestions.org