Charles Ellicott Commentary Amos 5:20

"Shall not the day of Jehovah be darkness, and not light? even very dark, and no brightness in it?" — Amos 5:20 (ASV)

Darkness: In the form of an interrogative, the condemnation contained in Amos 5:18 is emphatically reasserted. The term rendered “very dark” is that used to denote the gross Egyptian darkness that might be felt (Exodus 10:22), the awful gloom, such as fell on Jerusalem at the Crucifixion, which is always accompanied by the sense of confusion, terror, and intolerable suspense. (Compare to the graphic metaphorical language of Isaiah 5:30.)