Day of the Lord. 1st Thessalonians 5:2 This title would have been familiar to the Jews in the assembly at Thessalonica. The Old Testament scriptures refer to God’s just judgment against people who sin and to the future time of God’s reign on earth. Then, it will be a time of blessing. “Day” in this context does not mean a twenty-four-hour period but a span of time during which events happen one after another and will impact the entire world.
This long period of divine activity includes the Lord's return, just judgment, a great blessing, and full redemption. It will begin as unexpectedly as a “thief in the night,” even though there are signs that predict upcoming events and alert us that they will occur. For some people, the day of the Lord will be one of light and joy, while for others, it will bring darkness and sorrow.
Early believers knew that the day of the Lord was coming but did not realize that the start of that day would be the rapture of the church. That event will be the first act of the day when the judgment of sin begins on this earth. The punishment for those who rejected their Messiah, persecuted God's chosen earthly people, and knowingly and deliberately rejected Christ as their personal Savior will begin at the Lord's coming for His church. It will conclude when Jesus Christ reigns as King.
This was common knowledge from the scriptures, but the coming of the Lord for those who are His own was not known. This was the first written message regarding the rapture of the church. The manner of His coming is like "a thief in the night," not the time of His coming. As believers, we should maintain the kind of conduct and purity of life so that we will not be ashamed of what we are doing when He calls us to Himself.