Joel predicts that the period of the outpouring of the Spirit (which he has just described) will include a time of divine judgement for all nations. The judgment is described as taking place in a valley of Israel, although to look for such a location would be foolish because no valley anywhere could accommodate every person on earth. Instead we should realise that here we have a prophecy of the final judgement.
The standards of the judgement are how the nations treated God’s people (3:2-3), which is the same standards as Jesus says will be used on the Day of Judgement (Matt. 25:31-46). In verses 4-7 the prophet mentions specific sins committed by Tyre, Sidon and Philistia who will discover that their punishment is linked to their particular sins. What happened to those people in history becomes a picture of what will happen to all kinds of people on the Day of Judgement.
Verses 9-15 make clear that the Day of Judgement will not be a docile affair. Instead it will be the final battle between the Lord and his enemies. Even although the enemies make many weapons and even although they appear in large numbers they will not prevent the Lord from judging them. Joel likens the scene to a great harvest ingathering, except that it is an ingathering to judgement. The millions of the human race will be gathered to hear the irreversible decision of the Judge, and his voice when he speaks will shake the universe.
Yet on that awesome day the Lord will be the protector of his people. They will discover then what full salvation means (v. 16). A marvellous future awaits them as they live in the presence of God and enjoy his provision (vv. 17-18). Never again will they be harassed by their ancient enemies because God has judged them (vv. 19, 21). Instead his people will live forever in God’s country, enjoying the riches of his grace throughout the endless ages to come (v. 20).
Joel himself is still waiting in God's presence for his prophecies here to be fulfilled. But just as his prophecies about the Day of Pentecost came to pass, so too will his prophecies about the Day of Judgement and afterwards.