Thought from Today's Old Testament Passage:
[Joel 1:15] The day of the Lord is much too complex a subject to be contained in a brief discussion, for the prophets found it a most engaging topic in their ministries and writings. But it does not completely defy description. It will be a time when God judges the wicked as never before and simultaneously completes the salvation and deliverance of the redeemed.
It will also be a day of theophany or the appearance of God. He will appear on the Mount of Olives, just where the Son of God told the men from Galilee that he would come again in like manner as they had seen him go (Zech 14:3-4). When he appears Jerusalem will be attacked and he personally will lead his people against the nations that have gathered there to settle the Jewish question once and for all (Zech 14:3-12; Joel 3).
Because of this insurrection against the Lord and his cause, Amos describes this day as one of darkness and mourning (Amos 5:18-20) for those who had only the popular concept that the Messiah would return to magically right all wrongs for everyone, regardless of the person's personal belief in him. Amos went on to show how false this hope was.
Therefore, the day of the Lord was a term rich in content. It marked off a divinely inaugurated future period, which already had in some sense begun with the ongoing history of the kingdom of God.
Walter C. Kaiser, Hard Sayings of the Old Testament (Downer's Grove, IL: InterVarsity Press, 1988), 224-225