“About the times and the seasons: Brothers, you do not need anything to be written to you. For you yourselves know very well that the Day of the Lord will come just like a thief in the night” (1 Thessalonians 5:1-2 HCSB).
There are two references from 1 Thessalonians 5:2 that also appear within the New Testament epistle of 2 Peter…
“But the day of the Lord will come as a thief in the night, in which the heavens will pass away with a great noise, and the elements will melt with fervent heat; both the earth and the works that are in it will be burned up” (2 Peter 3:10).
In this context, the “day of the Lord” refers to the eventual dissolution of what we would call “the universe” or “space.” We also find similar expressions of this idea in the book of the Old Testament prophet Isaiah (Isaiah 65:17) and the New Testament book of Revelation (Revelation 21:1). Although this may seem disconcerting, these future realities should prompt us to make good, God-honoring choices in the present.
You see, the Apostle Peter goes on to ask (and answer) an important question in the following verse of 2 Peter chapter three. In view of the fact that everything we now possess will ultimately be dissolved, Peter arrived at an appropriate conclusion under the inspiration of the Holy Spirit: “Since everything will be destroyed in this way, what kind of people ought you to be? You ought to live holy and godly lives” (2 Peter 3:11 NIV).
In another sense, “the day of the Lord” arrives in an untold number of human lives every day. For instance, we might say the day of the Lord occurs whenever someone passes from this life into eternity. For some, that day arrives “like a thief in the night” in the form of an untimely passing. In those instances, the day of the Lord did not represent a far-off future event. Instead, it became the day when someone was unexpectedly called to stand before his or her Creator with no further opportunity to go back and undo the choices of life.
It’s often been said that the key to eternity is not to be ready when, but to be ready whenever– and as the Biblical book of Romans tells us, “If you openly admit by your own mouth that Jesus Christ is the Lord, and if you believe in your own heart that God raised him from the dead, you will be saved” (Romans 10:9 Phillips). Therefore, “Knowing how to live in preparation for the Lord’s return is more important than knowing the timing of His return (Acts 1:6–7).“ (1)
See related discussion beginning here
(1) Barry, J. D., Mangum, D., Brown, D. R., Heiser, M. S., Custis, M., Ritzema, E., … Bomar, D. (2012, 2016). Faithlife Study Bible (1 Th 5:1). Bellingham, WA: Lexham Press.