"The Day of the LORD” is a biblical term and theme used in both the Hebrew Bible (יֹום יְהוָה Yom Adonai) and the New Testament (ἡμέρα κυρίου, hēmera Kyriou), 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 which is read on the Sabbath of Vision, immediately before the 10th of Av.
The prophet Malachi foretells the return of Elijah immediately preceding the "great and terrible day of the LORD".[1] This prophecy is read in synagogues on the Great Sabbath immediately preceding Passover.
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[2] 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.