That day is the day of Adonai YHWH of hosts, a day of vengeance, to avenge himself on his foes. The sword shall devour and be sated and drink its fill of their blood. For Adonai YHWH of hosts holds a sacrifice in the north country by the river Euphrates.
Jeremiah 46:10

The Day of YHWH is the same as the day on which Israel's enemies appear to be most powerful. He uses them to destroy each other. God elevates the wicked in order to bring them down and make his own glory shine even more.