Joel was written during the locust plague, a time of great distress for the people. The prophet sees in the devastation of the locusts an indication of the coming of the day of the Lord. Therefore all must repent with fasting and mourning. With repentance, however, there is a promise of relief and God’s blessing for Israel.

Joel 2

2:12 Therefore also now, saith the LORD, turn ye even to me with all your heart, and with fasting, and with weeping, and with mourning: