(Isa. 13:6–9; 1 Ne. 11:35–36; Isa. 2:12; Amos 5:18; 3 Ne. 25:5–6)
The phrase “day of the Lord” first appears in the writings of the eighth-century prophets Amos and Isaiah (Amos 5:18–20; Isa. 2:6–22) to refer to a day of judgment that is so extremely severe that the people will howl with fear. Isaiah compares the trembling fear and shaking terror of the men to the anguish of a woman in labor… .
Verse 9 explains that the purpose of the day of the Lord is to purge the earth of all sinners. (See D&C 133:50–51; Joel 2:1–2; Mal. 4:1; 2 Thes. 1:7–9.) In his later writings, Isaiah elaborates upon the intensity and completeness with which the Lord will cleanse the earth (Isa. 24:1–6; 34:2–8; 63:4; 64:1–2; 66:15–16).
(Victor L. Ludlow, Isaiah: Prophet, Seer, and Poet [Salt Lake City: Deseret Book Co., 1982], 184.)
In general, this is that period of time just preceding the return of the resurrected Lord to this earth, as well as the time of His actual coming… .
Although the righteous will ultimately be saved in the kingdom of God, it should be pointed out that even some of them will fall prey to the war and pestilence that will precede the Second Coming. “It is a false idea,” said the Prophet Joseph Smith, “that the Saints will escape all the judgments, whilst the wicked suffer; for all flesh is subject to suffer, and ‘the righteous shall hardly escape;’ still many of the Saints will escape, for the just shall live by faith; yet many of the righteous shall fall a prey to disease, to pestilence, etc., by reason of the weakness of the flesh and yet be saved in the Kingdom of God.” (HC 4:11.)
(Hoyt W. Brewster Jr., Isaiah Plain and Simple [Salt Lake City: Deseret Book Co., 1995], 127–28.)