1 Thessalonians 5:2
"For yourselves know perfectly that the day of the Lord so cometh as a thief in the night."
“Day of the Lord”
is an all-encompassing term to describe the period that begins with the Great Tribulation, and includes the second coming of Christ and the millennial kingdom.
This Old Testament expression is here identified with the parousia already introduced. It will begin unexpectedly (Matt. 24:37).
There are 19 indisputable uses of “the Day of the Lord” in the Old Testament and 4 in the New Testament (Acts 2:20; 2 Thess. 2:2; Peter 3:10).
The Old Testament prophets used “Day of the Lord” to describe near historical judgments (Isa. 13:6-22; Ezek. 30:2-19; Joel 1:15; Amos 5:18-20; Zeph. 1:14-18), or far eschatological divine judgments (see Joel 2:30-32; 3:14; Zech. 14:1; Mal. 4:1, 5). It is also referred to as the “day of doom” and the “day of vengeance.”
The New Testament calls it a day of “wrath,” day of “visitation,” and the “great day of God Almighty” (Rev. 16:14). These are terrifying judgments from God (Joel 2:30-31; 2 Thess. 1:7-10), for the overwhelming sinfulness of the world. The future “Day of the Lord” which unleashes God’s wrath falls into two parts:
(1) The end of the 7 year tribulation period, (Rev. 19:11-21); and
(2) The end of the millennium.
These two are actually 1,000 years apart and Peter refers to the end of the 1,000 year period in connection with the final “Day of the Lord” (2 Peter 3:10; Rev. 20:7-15).
Here, Paul refers to the aspect of the Day of the Lord,” which concludes the tribulation period.
Jesus explains in the following Scripture that no man knows the exact time of the return of Christ.
Matthew 24:36 "But of that day and hour knoweth no [man], no, not the angels of heaven, but my Father only."