Joel threatens the Day of the Lord (which is code for both when God acted and when God will act decisively). In the prophets, this phrase is always bad. It symbolized how and why God is coming to judge and destroy. Joel clearly is warning Israel to repent from their sins and it appears they do we hear Joel say that God is gracious and compassionate, slow to anger and full of love. This language is the golden calf story from Exodus. Even in the face of sin, when God’s people repent, God reveals God’s mercy.