Amos again challenges the Israelites' mistaken belief about their chosen status (see Amos 3:2).
The phrase What will the Day of the LORD be for you indicates despair from a great tragedy.
In the Old Testament, the day of the Lord (see Isaiah 13:6, 9) was when God would intervene to fix what was wrong. God's intervention would mean vindication for the righteous and judgment for the wicked. Israel believed that on that day (see also Amos 8:3, 9, 13; 9:11), God would save them.
However, because the Israelites had been wicked, the day of the Lord would bring darkness, not light. Assyria conquered the northern kingdom in 722 BC (2 Kings 17:7–23), fulfilling this prophecy.