How easy it is to misunderstand someone by taking a phrase that means one thing and interpreting it differently. Sometimes that is because the person uses the phrase ironically or sarcastically. “Love that dress” could mean that the person thinks it is a really nice dress. When said differently it may mean that the person really hates it. Written or texted communication makes it worse, because voice inflection that usually conveys meaning is lost. At other times, the listener may just have a different perspective based on what they have been told by someone who didn’t understand the phrase. That seems to be the case when people today use “the day of the Lord.”
Many times when you hear that phrase today, people are talking about the end of the world. Of the 23 times the phrase is used in scripture, maybe three have that meaning. But because those who hold to some of the various premillennialist or “rapture” theories use the phrase exclusively that way, many people believe it must be interpreted that way every time.
A similar phrase, “the Lord’s day,” is used only once, and that in Revelation 1. Almost nobody interprets that in the same way, most taking it rather to mean Sunday. If anything, John most likely meant it to mean Saturday. At least one person, however, has read the verse as “I was, in the Spirit, on the Lord's day, and heard behind me a great voice, as of a trumpet.” The added commas around “in the Spirit” make it read that John’s vision actually took place “on the Lord’s day” regardless of what day he actually saw the vision. This person, then, believed the vision that is the book of the Revelation to be at the end of the world or at some significant event in Roman history later than John wrote. That person takes “the Lord’s day” to be equivalent of “the day of the Lord.”
When the prophets used that phrase, what did they mean? Only five of the uses of the phrase come in the New Testament, and one of those is a quote from one of the prophets. In the Old Testament, the 18 times it is used, the phrase only appear in the writings of the prophets. It never appears earlier than the book of Isaiah (or possibly Joel or Obadiah if you give them the early dates for their writing). It never appears in the historical or poetic works. So if we can understand how the Old Testament prophets used “the day of the Lord” we can get a more accurate understanding of its meaning.
Eight times the prophets say that the day is “near” or “at hand.” Of course, to some people that means nothing because they think the Revelation is in our future even though five times it says its fulfillment is near or at hand. But to a normal-thinking person, the idea is that the day may have had several fulfillments over time, none of which were in our future. If so, then what is that day? It is a day of God’s judgement. The terms the prophets use include “destruction” and “to lay desolate” (Isa 13; Joel 1), “vengeance” (Jer 46), “darkness and not light” (Amos 5), and “dreadful (Mal 4). It will come upon the proud (Isa 2); sinners (Isa 13), God’s adversaries (Jer 46), and the heathen (Obadiah). While this could apply to the end of the world, the contexts of these prophetic utterances show that it applies to many times. Obadiah, for instance, was speaking against the Edomites in what is now Petra, Jordan.
Joel uses the phrase four times, more than any other prophet. Peter quoted him on the first Pentecost after Jesus’ death. Does he say that Joel is speaking of the destruction of the physical world? No. He says he is speaking of the beginning of the church. The day of the Lord, at least in that instance, arrived almost two thousand years before today. That same Peter later used the phrase, perhaps, to mean the destruction of the physical world. Even then, though, he was using it as an extension of what began on that Pentecost.
Ultimately, then, the day of the Lord may be any day the Lord chooses to judge sinners. It may be individual (the day of death) or collective (when a government chooses to disobey). The final day will be the end of the world, but there may be many days of the Lord before then.