The Day of the Lord is the day when Jesus returns to bring both judgment and salvation. Paul reassures the Thessalonians that while that Day will happen unexpectedly and that it will be inescapable, they have nothing to fear because they are safe in Christ. And so, in light of that coming Day, Christians are to maintain situational awareness by knowing what time it is and they are to exercise self-control by holding on to faith, love, and hope.