The exact date of the book of Joel is difficult to know, since it does not refer to the reign of any particular king. The specific occasion of the book, however, is very clear and Joel uses this occasion to deliver a powerful spiritual message.
The book begins by graphically describing how a swarm of locusts has overrun the land of Judah and eaten everything in sight. After calling for the people to repent in response to this disaster, the book offers a detailed description of the locust swarm itself. The locusts are like an invading army, with God at their head. Joel asserts that the day of the Lord—a day of judgment—has come. He renews his call for repentance through fasting, community prayer and heartfelt contrition. In response, he promises that God will not only drive the locusts away, but restore more than they have devoured. He foretells God’s defeat of all the nations that oppose his people, and how God will pour out his Spirit on the survivors in Judah. If the people return to the Lord with all of their hearts, they will see the return of their prosperity when the day of the Lord arrives.