Book of Joel
It is hard to know when the Book Joel was written since the events told about may have happened many times. Most modern students of the Bible believe the locusts were real insects that destroyed the crops, leaving the people hungry. Others think they are a picture of big armies that will invade the land and destroy. They can also be both things, one past, the other future. Joel calls on the leaders and people to turn from their sins and ask God to forgive them. God promises to do that both at that time and in the future whenever similar punishments are needed.
The phrase "day of the Lord" is used by Joel to refer to a period when God will punish, but then forgive his people. Peter, one of Jesus' followers, talked about some verses from Joel to describe what happened on the day of Pentecost when Christians believe that the Holy Spirit was given and Christians talked about Jesus in languages they had never learned. This led to many people becoming Christians. This is found in the Book of Acts in the New Testament
The great army of locusts advances like darkness spreading over the mountains (2:2)