The prophet is explicit: The Day of the Lord is totally dark. There is nothing at all light about it!
It is important that we recognize the context of this passage. In verse 1, Amos terms his words “a funeral song—that I am lifting up against you, house of Israel” (Common English Bible). The Modern English Version and The Voice actually refer to it as a “dirge.” The King James Version uses the noun “lamentation.” With that definition in mind, notice the verb tenses in verse 2: “She has fallen; virgin Israel will never rise again. She lies abandoned on her land, with no one to raise her up” (Holman Christian Standard Bible).
Amos' vision is so clear that he is actually treating his subject, the nation of Israel, as though she were already dead—gone. Yet, he wrote these words some forty years before ten-tribed Israel (that is, the Northern Kingdom) had actually fallen to the Assyrian Empire. Furthermore, we know that Israel will rise again, when God restores her, joining her again with Judah (Ezekiel 37:15-28). Indeed, Amos himself speaks of this restoration in Amos 9:11-15. Notice just verses 14-15:
“I will restore the fortunes of my people Israel, and they shall rebuild the ruined cities and inhabit them; they shall plant vineyards and drink their wine, and they shall make gardens and eat their fruit. I will plant them on their land, and they shall never again be uprooted out of the land that I have given them,” says the LORD your God. (English Standard Version)
Both Old and New Testaments speak of this restoration. Clearly, the language of Amos 5:2 is hyperbolic (that is, overstatement), highly apropos rhetoric for a dirge, the rhetoric of which is about as dark as it can be. With that context in mind, Amos' meaning in verses 18-20 becomes clearer. The prophet is in fact saying that the people of Israel thought the Day of the Lord was one of total light. They misunderstood.
If we were to transport a representative cross-section of Amos' audience to the twenty-first-century America, we might, after interviewing them, discover a lot of common ground between these self-righteous and hypocritical Israelites and the post-Millennialist members of today's liberal churches. Post-Millennialists believe that “things” are improving all the time, the result of the effective work of the church. They believe that, eventually, things will be so good that Christ will return. It is almost as though these people listen to different newscasts than the ones to which we listen!
So, too, the Israelites of Amos' day, focusing myopically on their current wealth and false sense of wellbeing, perceived nothing but “good times rolling.” Times were great, getting greater, with no end to prosperity in sight. They imagined themselves to be at the gate of Paradise, what they thought the Day of the Lord would be.
Amos corrects that errant perception. The time is coming, he avers in Amos 5:16, “In all the squares there shall be wailing, and in all the streets they shall say, 'Alas! Alas!'” This is a far cry from “good times”! From their distress, he says in verse 19, they will find no viable path of escape—running from a lion, they meet a bear! Their doom is sealed.
In reality, for those doomed, the Day of the Lord will have no good in it at all. It will be totally dark, exactly the opposite of what Amos' audience dimwittedly envisioned. We know, however, that those not doomed during the Lord's Day will see God promptly take restorative action, extending “great compassion” to them.