For you yourselves know accurately that the day of the Lord will thus come as a thief in the night (1 Thessalonians 5:2).
The day of the Lord in this context refers to Christ’s return to the earth to establish His kingdom and claim His inheritance. After the two witnesses in Jerusalem are finally killed by the man of lawlessness—the beast—those who dwell on the earth will rejoice and exchange gifts with one another, saying, “Peace and safety” (Revelation 11:7–10). At that time, sudden destruction will come upon them as the Lord returns.
We are not of the darkness; therefore, that day will not overtake the grace believer (1 Thessalonians 5:3). The Church will be with Christ in the heavens and will return with Him as He crushes the nations and establishes His kingdom, reigning over the world in righteousness for a thousand years before the final judgment comes upon all the unsaved (Jude 14; Revelation 20:6; Matthew 13:41). Afterward, a new heaven and a new earth will be made, in which righteousness settles down and feels at ease (2 Peter 3:13).