Joel uses a present catastrophe — a locust plague — to point to a greater "day of the LORD" still to come, both terrible and hopeful. The book promises that God will pour out his Spirit on all flesh, a prophecy Peter quotes at Pentecost.
“And rend your heart, and not your garments, and turn unto the LORD your God: for he is gracious and merciful, slow to anger, and of great kindness.”
“And it shall come to pass afterward, that I will pour out my spirit upon all flesh; and your sons and your daughters shall prophesy.”