In scripture, the "Day of the Lord" indicates a great move of God, this time in judgement. God had directed his army of locusts to invade Judah. Despite the devastation already caused, the worst was yet to come. This coming day of the Lord would see the land turned into something like a post-apocalyptic waste ground. The only way to avoid this culmination of God's wrath was through repentance, and this principle applies to the modern church as much as the ancients.
