Rhetorically Amos stated that the coming day of the Lord would be a day characterized by darkness and gloom (despair) rather than by bright light (joy; cf. Joel 2:1-2; Joel 2:10-11; Zephaniah 1:14-15). [source][source][source]
A brighter day of the Lord was also coming (cf. Amos 9:11-15; Jeremiah 30:8-11; Hosea 2:16-23; Micah 4:6-7; Zephaniah 3:11-20), but first a dark one would appear. The Israelites wanted to hasten the good day of the Lord, but they wanted to forget about the bad one. This prophecy found fulfillment when the Assyrians overran Israel and took most of the people into exile in722 B.C. The later Tribulation period for Israel, which will precede her millennial day of blessing, will be similar to what Amos predicted here, but I think it was not what God was foretelling here. [source][source][source]
Context Summary
Amos 5:16-27 - A Dark Day For Hypocrites
Mighty sins had been committed, and mighty judgments were at hand. The oppression of the poor, Amos 5:11; the erection of elegant dwellings from unrighteous exactions, Amos 5:11; the acceptance of bribes to betray the needy, Amos 5:12 all these must be reckoned with. But if the guilty nation would not seek God and establish judgment in the gate, where magistrates sat to dispense justice, the streets would be filled with wailing, and the husbandmen and vine-dressers would be equally affected by the widespread desolation as the dwellers in the cities.
Moreover, bad as Israel's present condition was, it would become infinitely worse, as though a man fleeing from a lion rushed into the arms of a bear, or, taking refuge in a house, was stung by a serpent that lay hid in a cranny of the wall. Of what avail are religious rites, when the heart is alienated from God, Amos 5:21, etc.? Let us heed well the exhortation of Amos 5:23-24. The martyr Stephen quoted Amos 5:25-27, which accuse the people of carrying about little shrines and pocket-idols, to serve as amulets averting disaster, Acts 7:43. But they might as well have built a bank of sand to arrest an overflowing flood! The one thing which is going to help us is repentance toward God and faith in our Savior Jesus Christ. [source]
Chapter Summary: Amos 5
1A lamentation for Israel 4An exhortation to repentance 21God rejects their hypocritical service
What do the individual words in Amos 5:20 mean?
[Is] notdarknessthe dayof Yahwehand notlightand [Is it not] very darkand nowith brightnessin it