The first thing that may stand out if you read Joel's prophecy is that his main subject seems to be "the day of the Lord". He mentions the day of the Lord six times in three short chapters, and twice he describes it as “great and terrible”.
Joel began his prophecy by describing in detail a devastating locust plague that had recently destroyed the land of Judah. He describes what the locusts had done to the fields, the grain, the vineyards, the trees, and how the people had been affected. He saw the locust plague as God's judgment on Judah and called the people to repent. The locust plague, he said, was a warning of greater judgment to come unless the people turn from their sin and return to God with all their hearts (Joel 2:12-13). It won't be enough to just say "sorry", expect God to forgive their sin, and then go on living the same sinful lives, following the ways of the pagan nations and worshiping their gods rather than following and living for the one true God.
So Joel grabs the attention of the people, and hopefully our attention as well, by saying this locust plague is just a small taste of the great and terrible day of the Lord that is coming. Then He describes that day. There will be signs in the heavens and the heavens and earth will be shaken. It will be a solemn day of judgment. God will judge all nations (and people) who have denied and defied Him. All those who have made the decision to rebel against Him and refused to repent will be gathered to the "valley of decision" where they will receive God's decision of judgment. But the day of the Lord will also be a day of grace for God’s people. Those who have truly repented of their sin and love and obey God will not be judged but will rejoice in the strength, security, and blessings of God.
Many people who read Joel's prophecy get caught up in the drama of the day of the Lord and search the Scriptures to find out more or read books to see what others have to say about it. This is all well and good unless they forget to look for the main point Joel is making. His reason in proclaiming the day of the Lord is to give us the opportunity to make sure we will not be among those being judged on that day. The heart of Joel's prophecy is in Joel 2:12-14, his call to turn to the Lord with all your heart, rend your heart and not your garments, and repent and return to Him. God is gracious and merciful and does not desire that any should perish (2 Peter 3:9), but He is also holy and just. And while He may allow evil to continue for a while, He will not tolerate it forever.
The day of the Lord is coming. We don't know when it will be. Joel didn't know. None of the prophets knew. Jesus told His disciples that no one knows except God (Matthew 24:36). The apostle Paul wrote, in his first letter to the Thessalonians that the day of the Lord will come like a thief in the night and take many by surprise (5:2-3). But he also wrote, God did not appoint us to wrath, but to obtain salvation through our Lord Jesus Christ (5:9). Joel, hundreds of years before Paul wrote, told us how to make sure we do not face God’s wrath in judgment. Joel made clear that regret and remorse is not enough when it comes to dealing with sin. Repentance is needed. Joel says repentance is not just throwing out your old clothes and putting on clean ones. God calls for true repentance, which means grieving over your sin, turning to Him and giving Him your heart to cleanse and change so you can love Him, follow Him, and obey Him with every fiber of your being. Repentance begins deep inside us.
As I was studying in preparation for writing a new Disciplers study on Joel, I came across these thoughts on the day of the Lord by Jack Wellman (patheos.com): “Today is the best day to repent and be saved if you haven’t already done so, for the Day of the Lord could come today while you’re reading this, tonight in your sleep, or if you were to die, the very next moment of your consciousness.”
My prayer is that you will take Joel’s message to heart, and use it, as the writer of Hebrews says, to encourage on another daily, while it is called "Today", lest any of you be hardened through the deceitfulness of sin (3:13).