"Behold, I will send you Elijah the prophet before the coming of the great and dreadful day of the LORD." (Malachi 4:5)
This is the next-to-last verse of the Old Testament and so marks the final mention in the Old Testament of the fearsome theme of the Day of the Lord. As the text says, it will be a "great and dreadful day."
This phrase occurs frequently in the Bible, reminding us over and over again that although God is merciful and longsuffering, He will not remain silent forever. Man's "day" will end someday, and the day of the Lord will come.
Note some of the other prophecies: "Woe unto you that desire the day of the LORD! . . . the day of the LORD is darkness, and not light" (Amos 5:18). "The sun shall be turned into darkness, and the moon into blood, before the great and terrible day of the LORD come" (Joel 2:31). "The great day of the LORD . . . is a day of wrath, a day of trouble and distress, a day of wasteness and desolation, a day of darkness and gloominess, a day of clouds and thick darkness" (Zephaniah 1:14-15). "Behold, the day of the LORD cometh, cruel both with wrath and fierce anger" (Isaiah 13:9).
The phrase also is repeated in the New Testament, most awesomely of all in 2 Peter 3:10: "The day of the Lord will come as a thief in the night; in the which the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also and the works that are therein shall be burned up" (see also 1 Thessalonians 5:2, etc.).
Without trying to sort out the precise sequences and events associated with all such prophecies, it is obvious that the Day of the Lord is a coming time of terrible judgment on all who have rejected or ignored the God who created them. But God's faithful believers can take great comfort, for then "the LORD shall be king over all the earth: in that day shall there be one LORD" (Zechariah 14:9). HMM