The Great Contrasts
Paul had assured the Thessalonians in 4:13-18 that the dead in Christ will rise and we who are alive and remain will be caught up together with them in the clouds to meet Jesus. Now in 5:1, Paul continues to talk about end time events but his focus shifts from the rapture of the church to the “day of the Lord.” The subject is similar but different. The key difference in what he has to say here is not so much "when" these events take place but the "whom" that is involved. The big difference here is that the rapture deals with the saved and the “day of the Lord” is going to impact the lost.
Paul seeks in this section to contrast the differences between the saved and lost in terms of how we view the second coming of Christ. How Christians view the second coming and how the lost world views the second coming are totally different. How each group will be impacted by the second coming will be totally different. There is great contrast.