Of the minor Prophets, Joel is judged to be the earliest in connection with Judah, though there are no dates given in the prophecy itself. The key-note of the prophecy is 'the day of Jehovah,' which is five times mentioned in connection with the future judgements, which will bring in the full blessing of Israel and the earth, when the Lord also will have His portion, a meat offering, and a drink offering for Himself.
The Prophet takes occasion by the devastation wrought in his day by an army of insects to call the priests, the princes, and the people to a fast, and a solemn assembly in the house of the Lord, there to cry unto Jehovah. Then he adds, "Alas for the day! for the day of the Lord is at hand, and as a destruction from the Almighty shall it come." Here it is destruction, open judgement, as in the day when God will judge the world in righteousness. The army of insects was but a precursor, but as a present thing, instead of joy and gladness being in the house of God, God was judging. The prophet said 'is at hand;' but God's long-suffering deferred its full execution, and defers it still.
The day of Jehovah is nigh at hand, and the trumpet is to sound an alarm of war: cf. Num. 10: 9. The army of insects is still alluded to, but it looks forward to the future, when God will bring His judgements upon the land. The army is His, and the camp is His: the day of Jehovah is great and very terrible. The people are called to repentance, to rend their hearts and not their garments, for God is merciful and gracious. The trumpet was to be blown in Zion for a solemn assembly: cf. Num. 10: 7. Priests and all are called to weep and pray. God will hear, and will destroy their enemies, especially the northern army (Joel 2: 20, elsewhere alluded to as Assyria) and He will bring His people into great blessing. When they repent, the Holy Spirit will be poured out upon them and upon all flesh. This was quoted by Peter in Acts 2: 16-21, but the nation did not then repent, it was only a remnant that turned to the Lord and entered into the blessing that God was bestowing - not outward and visible benefits as it will be in the future. There will also be signs in the heavens and in the earth before the great and terrible day of the Lord. There were some such omens, according to the historians, before the destruction of Jerusalem, so this passage, quoted in Acts 2, may have had a partial fulfilment then, though it remains to be fully verified in a future day.
This enters into the details of the last days as far as Judah and Jerusalem are concerned, the restoration of the ten tribes not being the subject here. The nations have oppressed God's people in many ways, and sold them as slaves. God will requite this on their own heads. They are called to arm themselves, to bring all their mighty men, and to come unto the valley of Jehoshaphat, which is the valley of judgement, and there God will deal with them. In the valley of decision (or threshing) they will be cut to pieces. The enemies of God and of Judah being destroyed, there will be great blessing for His people, whom He had chastened in His love; but, cleansed and restored, He will dwell among them.