The phrase "day of the Lord" occurs thirty-one times in the King James Bible. It is found twenty-six times in the Old Testament and five times in the New. Its first mention is in Isaiah 2:12 and its last recorded use is in 2Peter 3:10. The day of the Lord takes place on the earth and occurs roughly during the last eighteen months of the great tribulation period.
The day of the Lord will be the most concentrated time of trials, pain, suffering and mass death the world has, or ever will, experience (Matthew 24:21 - 22). Paradoxically, it will also be a period of unprecedented wealth and prosperity for at least some of humanity (see Revelation 18:9 - 19).
Satan, during this "day," continues his concerted effort to hunt down and kill anyone who does not exclusively obey and worship him (Revelation 6:11, 13:10). His authoritarian rule continues to be manifested through the Beast and False Prophet.
At the same time, however, the Lord's presence in the affairs of mankind will be undeniably evident to all. God, during this prophetic day, will be actively encouraging people to repent and punishing those who do not.
Revelation's sixth seal
The opening of Revelation's prophetic sixth seal will set off a series of supernatural events that will warn all humans that the day of God's correction will soon commence (Revelation 6:17). Earthquakes will occur in various places as all the planet's continental plates violently shift. The sun will darken and the moon will appear to look like blood. Stars commonly seen at night will disappear from the sky (Joel 2:31, Matthew 24:29, Acts 2:20, Revelation 6:12 - 14).
144,000 people or twelve thousand from twelve of Israel's tribes (the tribe of Dan excluded) become true Christians and are sealed or set apart (Revelation 7:1 - 8). They die for their faith at the hands of the devil's servants and ultimately receive their rewarded in the first resurrection. In addition, a great multitude from around the globe are brought to repentance and die a martyr's death (verses 9 - 17).
Sometime after the sixth seal is opened, but before this prophetic day ends, God will pour out his Holy Spirit upon those who seek him. They will miraculously be given prophetic visions and dreams (Acts 2:17 - 21, Revelation 6:12 - 16).
Trumpets, Woes and Vials
This prophetic day begins when a seventh seal is opened in heaven (Revelation 8:1). This seal is unique in that the first event it kicks off is a short silence in heaven, possibly as recognition of God beginning his long awaited judgment of Babylon and those she has deceived to sin (14:8, 16:19, 17:5, etc.).
The seventh seal that starts the day of God's wrath also reveals seven additional prophetic trumpets that will blow. The first four of these trumpets announce a variety of calamities to come upon mankind.
For example, one third of the earth's vegetation will be destroyed as well as one third of all water-based animals. Additionally, one third of all the natural supplies of fresh water, including that which is below ground level, will be poisoned and undrinkable (Revelation 8:3 - 12).
The last three prophetic trumpets to be blown during the day of the Lord's vengeance (Luke 21:22) are also Biblically called "woes" (Revelation 8:13). The first woe, which lasts a few months, is meant to punish those who still have refused to repent of their sins (Revelation 9:1 - 11).
The sixth trumpet (second woe) will be the releasing of four demons who were kept in a spiritual prison at the Euphrates River. Their release will cause the deaths of one-third of all humanity (Revelation 9:13 - 19)!
The final trumpet blown (third woe) announces, in heaven, that God will soon send Jesus back to the earth to establish his kingdom (Revelation 11:14 - 17). It will be a time when the first resurrection of all the saints takes place (verse 18). It is also revealed, however, that the day of the Lord will not be completed until seven more "vial" plagues come upon the unrepentant (Revelation 15 - 16).
Why is it needed?
Why is a Day of the Lord needed? Why does God inflict multiple punishments on a world in bondage to Satan's authoritarian and violent rule? The answer is simple. It is to lead humans to reconsider what they have done. It is to encourage them to accept the fact that they have been deceived and not done what is right, and repent!
The level of pain experienced in this day of wrath must be greater than mankind's determination to do evil in order to get his attention and have him repent. God's perfect love motivates him to do this because he wants as few as possible to suffer the same chaotic existence and ultimate fate of the devil.