"The sun and moon will be darkened, and the stars no longer shine. The Lord will roar from Zion and thunder from Jerusalem; the earth and the sky will tremble. But the Lord will be a refuge for his people, a stronghold for the people of Israel." NIV translation
The darkness that will occur prior to the return of Christ will be both physical and spiritual. As John described in Revelation 16, the whole world is plunged into darkness as the fifth angel pours out his bowl of wrath. We are reminded that it is Jesus who will defend His people and His city.
"'Then you will know that I, the Lord your God, dwell in Zion, my holy hill. Jerusalem will be holy; never again will foreigners invade her.'" NIV translation
As we see in Revelation 19, Jesus handles the attacking forces and it is obvious to all that it could only be through the power of God.
As Joel tells us here, the world will see that Jerusalem is the city of God and this will begin the period of time that is commonly called the Millennial Reign.
"'In that day the mountains will drip new wine, and the hills will flow with milk; all the ravines of Judah will run with water. A fountain will flow out of the Lord's house and will water the valley of acacias. But Egypt will be desolate, Edom a desert waste, because of violence done to the people of Judah, in whose land they shed innocent blood.'" NIV translation
This Millennial Reign will be a big contrast between the rest of the world and the land of God. The earth will have been decimated by the plagues and judgments but the city of Jerusalem will stand like an oasis in the desert. It will be clear to see the presence of El Shaddai (God Almighty) the only One who has the ability to supply what is needed.
"'Judah will be inhabited forever and Jerusalem through all generations. Their bloodguilt, which I have not pardoned, I will pardon.' The Lord dwells in Zion!" NIV translation
We see that, at the end of this Millennial Reign, will be the final judgment. The sins of God's people are forgiven through the blood of Jesus Christ and a new Jerusalem is revealed (see Revelation 21). At the center of the new Jerusalem will be the throne of God.