The day of the Lord

"The Day of the Lord" is a biblical term and theme used in both the Hebrew Bible (Old Testament; יֹום יְהוָה) and the New Testament (ἡμέρα κυρίου), as in "The sun shall be turned into darkness, and the moon into blood, before the great and the terrible day of the LORD come" (Joel 2:31, cited in Acts 2:20).

In the Hebrew bible, the meaning of the phrases refers to temporal events such as the invasion of a foreign army, the capture of a city and the suffering that befalls the inhabitants. This appears much in the second chapter of Isaiah.

In the New Testament, the "day of the Lord" may also refer to the writer's own times, or it may refer to predicted events in a later age of earth's history including the final judgment and the World to Come. The expression may also have an extended meaning in referring to both the first and second comings of Jesus Christ.

It is used first by Isaiah and subsequently incorporated into prophetic and apocalyptic literature texts of the Bible. It relies on military images to describe the Lord as a "divine warrior" who will conquer his enemies. In certain prophetic texts of the Old Testament, the enemies of the Lord are Israel's enemies, and in these visions the day of the Lord brings victory for the people of ancient Israel. Other prophets use the imagery as a warning to Israel or its leaders and for them, the day of the Lord will mean destruction for the biblical nations of Israel and/or Judah. This concept develops throughout Jewish and Christian Scripture into a day of divine, apocalyptic judgment at the end of the world.

...
Wikipedia