The reference to darkness and light is meant to contrast danger with safety.1 The day of the Lord will not be one of light and brightness (see Psalm 18:12, Psalm 18:28) which the Israelites imagined in song, but rather darkness and gloom like the plague in Egypt (Exodus 10:21–29).
20 Is not the day of the LORD darkness, and not light, and gloom with no brightness in it?