What does Isaiah 13:6 mean?

This is Isaiah's oracle against Babylon, describing a massive, then-future attack against the powerful city-state (Isaiah 13:1). The Lord has also made it inescapably clear that this is His attack against Babylon. He has assembled this massive army, consisting of the forces of multiple, far-flung nations. This attack will convey His anger in righteous judgment(Isaiah 13:3–5). The Lord is working out His purposes through this attack.

Isaiah tells the people of Babylon in this moment to "wail" or "howl." They should be humble enough at the prospect of this destruction from the Lord to be terrified. He writes that the "day of the Lord" is near. The day of the Lord is most often used in the Bible, especially by the Old Testament prophets, to describe when God's judgment will bring destruction on the guilty and salvation to the righteous.

The day of the Lord often refers to the ultimate time of God's judgment before the beginning Christ's kingdom on earth or of eternity. At other times, the day of the Lord may be more specifically directed at one nation or people for their specific sins. The latter seems to be the case here.