Joel 1:12 The vine is dried up and the fig tree languishes; the pomegranate tree, the palm tree also and the apple tree, even all the trees of the field, are withered, because joy is withered away from the sons of men.

Joel 1:15.Alas for the day! for the day of God is at hand and as a destruction from the Almighty shall it come.

Here God uses the Chaldeans, locusts and other insects in these desolations about to come upon the ancient land of Judah.

Joel 2:1 Blow you the trumpet in Zion and sound an alarm in my holy mountain. Let all the inhabitants of the land tremble, for the day of God comes, for it is nigh at hand;

Joel 2:2-10 A day of darkness and of gloominess, a day of clouds and of thick darkness, as the morning spread upon the mountains; a great people and a strong. There has not been ever the like, neither shall be any more after it, even to the years of many generations. A fire devours before them and behind them a flame burns. The land is as the garden of Eden before them and behind them a desolate wilderness, yea and nothing shall escape them. The appearance of them is as the appearance of horses and as horsemen, so shall they run. Like the noise of chariots on the tops of mountains shall they leap, like the noise of a flame of fire that devours the stubble, as a strong people set in battle array. Before their face the people shall be much pained. All faces shall gather blackness. They shall run like mighty men. They shall climb the wall like men of war and they shall march every one on his ways and they shall not break their ranks; neither shall one thrust another. They shall walk every one in his path and when they fall upon the sword, they shall not be wounded. They shall run to and fro in the city. They shall run upon the wall, they shall climb up upon the houses. They shall enter in at the windows like a thief. The Earth shall quake before them. The heavens shall tremble. The sun and the moon shall be dark and the stars shall withdraw their shining.

Joel 2:11 And God shall utter his voice before his army, for his camp is very great, for God is strong in executing his word. The day of God is great and very terrible.(of great fear, why?.Revelation 1:7).and who can abide it.(certainly not the evil ones)?

Joel 2:12 Therefore also now says God, turn you even to me with all your heart and with fasting and with weeping and with mourning.

2Chronicles 7:14.

Joel 2:13 And rend your heart.(deeply examine yourself and contemplate it).and not your garments.(in the Old Testament they showed how upset they may have become over something, by tearing their clothes; they weren't worried about 'how do I look for everybody', which would heve been an ego thing taking the mind away from self-examination).and turn unto the Lord your God, for he is gracious and merciful, slow to anger and of great kindness and repents himself of the evil.

The last thing God wants is to wake people up severely, but this often had to be done (captivities) with the stubborness of ancient Israel and ancient Judah and so the same it is with people today walking around with the same attitude:.Lamentations 3:33 "For he does not afflict willingly nor grieve the children of men."

God changes His mind (repents) toward those who want to change for the better and get away from their life of heartaches, many heartaches of which they have been deceived into, keeping them apart from a better way.

The word 'repent' simply means 'change', that is, to be of the heart to want to change. Why just 'be of the heart to want to change'?

Joel 2:14 Who knows if he will return and repent and leave a blessing behind him even an offering of food and drink from the Lord your God?

Verse is talking about a changed heart and the joy and actions of an individual who has discovered God.

Joel 2:15-18 Blow the trumpet in Zion, sanctify a fast, call a solemn assembly. Gather the people, sanctify the congregation, assemble the elders, gather the children and those that suck the breasts. Let the bridegroom go forth of his chamber and the bride out of her closet. Let the priests, the ministers of the Lord, weep between the porch and the altar and let them say, Spare thy people, O God and give not your heritage to reproach, that the heathen should rule over them.(but God did, verse 19 below). Wherefore should they say among the people, where is their God? Then will God be jealous for his land and pity his people. 

Joel 2:19 Yes God answers and says unto his people, Behold, I will send you corn and wine and oil and you shall be satisfied therewith and I will no more make you a reproach among the heathen.(it's the true God that did this and why?), 

Joel 2:20-22 But I will remove far off from you the northern army and will drive them into a land barren and desolate with his face toward the east sea and his hinder part toward the utmost sea and his stink shall come up and his ill savour shall come up.(dead putrefying bodies all around; the Creator protects those who look to Him), because he has done things that he thought in his greatness.(swelled in pride by his accomplishments). Fear not, O land; be glad and rejoice, for God will do great things. Be not afraid you beasts of the field, for the pastures of the wilderness do spring, for the tree bears her fruit, the fig tree and the vine do yield their strength.

Joel 2:23-25 Be glad then, you children of Zion and rejoice in God. For he has given you the former rain moderately and he will cause to come down for you the rain, the former rain, and the latter rain in the first month. And the floors shall be full of wheat and the fats shall overflow with wine and oil. And I will restore to you the years that the locust hath eaten, the cankerworm and the caterpiller and the palmerworm, my great army which I sent among you.

Joel 2:27 And you shall know that I am in the midst of Israel.(and who is Israel today?).and that I am your God and none else and my people shall never be ashamed.

Joel 2:28 And it shall come to pass afterward.(when? Acts 2:1-17), that I will pour out my spirit upon all flesh and your sons and your daughters shall prophesy, your old men shall dream dreams, your young men shall see visions.

Joel 2:29 And also upon the servants and upon the handmaids in those days will I pour out my spirit.

Joel 2:30 And I will shew wonders in the heavens and in the Earth, blood and fire and pillars of smoke. 

Joel 2:31 The sun shall be turned into darkness and the moon into blood, before the great and the terrible day of the Lord come.(Acts 2:20).

Joel 2:32 Call on the name of God and be delivered, for in mount Zion and in Jerusalem is deliverance, as God has said and in the remnant whom God shall call.

Those who will be called are also under protection.
Verse here is closer to the original than the King James translation has it.

Joel 3:1 For, behold, in those days and in that time, when I shall bring again.(original is 'bring out', that is take them out of captivity).the captivity of Judah and Jerusalem.

Joel 3:2 I will also gather all nations and will bring them down into the valley of Jehoshaphat and will plead with them there for my people and for my heritage Israel, whom they have scattered among the nations and parted my land.

Joel 3:4 Yea and what have ye to do with me, O Tyre and Zidon, and all the coasts of Palestine? will you render me a recompence? and if you recompense me, swiftly and speedily will I return your recompence upon your own head.

Joel 3:6 The children also of Judah and the children of Jerusalem have you sold unto the Grecians, that you might remove them far from their border.

Joel 3:3-9 And they have cast lots for my people and have given a boy for an harlot and sold a girl for wine, that they might drink. Yea and what have you to do with me, O Tyre and Zidon and all the coasts of Palestine? will you render me a recompence? and if you recompense me, swiftly and speedily will I return your recompence upon your own head, because you have taken my silver and my gold and have carried into your temples my goodly pleasant things. The children also of Judah and the children of Jerusalem.(who are they today).have you sold unto the Grecians, that you might remove them far from their border. Behold, I will raise them out of the place whither you have sold them and will return your recompence upon your own head. And I will sell your sons and your daughters into the hand of the children of Judah and they shall sell them to the Sabeans, to a people far off, for God has spoken it. Proclaim you this among the Gentiles; Prepare war, wake up the mighty men, let all the men of war draw near; let them come up.

Joel 3:10 Beat your plowshares into swords and your pruninghooks into spears. Let the weak say, I am strong.

Joel 3:11-15 Assemble yourselves and come all ye heathen and gather yourselves together round about. Thither cause thy mighty ones to come down, O God. Let the heathen be wakened and come up to the valley of Jehoshaphat, for there will I sit to judge all the heathen round about. Put you in the sickle for the harvest is ripe. Come, get you down, for the press is full, the fats overflow; for their wickedness is great. Multitudes, multitudes in the valley of decision. For the day of God is near in the valley of decision. The sun and the moon shall be darkened and the stars shall withdraw their shining.

Joel 3:16 God also shall roar out of Zion and utter his voice from Jerusalem and the heavens and the Earth shall shake, but God  will be the hope of his people and the strength of the children of Israel.

....next Old Testament book is Amos