The "Day of the Lord" refers to a future time when the Lord will act in history to punish his enemies and to save those who have been faithful. In the Old Testament, the day of the Lord (or simply, "the day") is described most often in the books of the prophets. It usually refers to some future historical event, rather than to a final judgment when God's enemies receive eternal punishment and God's faithful people receive eternal life. This idea of a final judgment day is more common in the New Testament.

The oldest passage in the Bible that mentions the Lord's day of judgment is found in Amos (Amos 5:18-20). This and other passages from the prophets describe the day of the Lord as a dark day of disaster when the Lord will punish the wicked. The "wicked" may be other nations (Isa 13:1-6; Amos 1:3-2:3; Obad 15), the people from Israel and Judah (Ezek 7:2-13; Joel 1:15), or the people of the earth in general (Isa 2:12-21; Zeph 1:14-18). Punishment connected with the day of the Lord can take the form of an invasion by an enemy nation (Isa 10:5-12; Hos 10:10-14) or natural disasters such as swarms of locusts (Joel 1:4-7; Amos 7:1, 2), earthquakes and darkness (Joel 2:10; Amos 8:9), and famine (Ezek 5:13-16).

The day of the Lord also is described as a time when God will restore the people of Israel and Judah and save those who have been faithful. This time of restoration follows a time of punishment. For example, the prophets Isaiah, Jeremiah, and Ezekiel each warned the people that their exile in Babylon was punishment for being unfaithful to the Lord. But the Lord would forgive them and lead them back to Jerusalem, where they could once again be a "light" among the other nations (Isa 40:1-11; 49:8-12; Jer 23:7, 8; Ezek 37:15-23; see also Amos 9:11-15). Often this time of restoration is described as a time of peace when the people would once again be ruled by a chosen ruler (messiah) from the family of David (Isa 11:1-9; Jer 23:5, 6).

A few Old Testament passages hint that God's saving love and judgment affect more than events in this life. For example, the prophet Daniel speaks of a time when the dead will rise from death. At this time, some people will receive eternal life and others will receive eternal shame (Dan 12:1-3). See also Job 19:25-27 and Isa 26:19. These passages and later texts that come from writings dating to the period between the Testaments (first and second century B.C.) laid the foundation for the many New Testament passages that describe a coming day of judgment.

The Gospels refer to a day of judgment (Matt 10:15; 12:36; Luke 21:34, 35), and Jesus describes a time of coming judgment that sounds a great deal like the disastrous day of the Lord described by the prophets (Mark 13). But his words introduce the idea that the coming time of disaster is a warning that the end of time is near and soon the Son of Man will return and gather his chosen ones (Mark 13:24-27). One of Jesus' parables describes a coming time of judgment that is meant to encourage people to help their neighbors in this lifetime (Matt 25:31-46).

When the apostle Paul speaks of the coming day of the Lord he connects it to the return of Christ (1 Cor 1:8; 5:5; Phil 1:6-11; 2:16; 1 Thes 5:2). Revelation describes the future day when God will finally defeat the forces of evil as "the day of God's great victory" (Rev 16:14). Death itself will be thrown into the lake of fire along with all those whose names are not written in the book of life (Rev 20:14, 15). Then God will create a new heaven and new earth (Rev 21:1-7). Some New Testament passages suggest that people face judgment immediately after they die (Luke 16:19-24; 23:39-43).