Joel 3

“Let the nations be wakened, and come up to the Valley of Jehoshaphat;
For there I will sit to judge all the surrounding nations.Put in the sickle, for the harvest is ripe. Come, go down; for the winepress is full, the vats overflow—For their wickedness is great.  Multitudes, multitudes in the valley of decision.  For the day of the Lord is near in the valley of decision.  The sun and moon will grow dark, and the stars will diminish their brightness." vv.12-15

Read chapter 3.
Well looks like from these verses we get a bit more of a picture of what the judgment will look like.  It will take place in the Valley of Jehoshaphat ("Yahweh is Judge"), which is between the Mount of Olives and Jerusalem.  There are a couple ideas why it's called this, possibly where King Jehoshaphat overthrew the enemies of Israel in what they called the Valley of Berachah (blessing) or wherein was also found Valley of Hinnom where a lot of idolatrous worship happened that then became the Valley of Slaughter from judgment of the Lord (Jeremiah 7:31).  Also we see happening during this judgment time that the sun, moon and stars grow dark and diminish in brightness.  This isn't necessarily not because there won't be sun, moon or stars or that it's a bad thing that they're "darkened," but that Christ's light outshines them.  What else do you see in this chapter describing that time?  What is new to you?