The Day of the Lord is an important term in the Bible, mentioned in 17 different books. Today it appears people are getting away with sin, but during the Day of the Lord, God’s judgment will be direct and dramatic. This period begins with judgment upon every aspect of society and moves to reward, darkness to light, curse to blessing.

I. The Times (1 Thess 5:1-3). Paul introduces a new subject here, shifting from the Lord’s catching away of His people to what happens when Jesus returns. Paul didn’t need to teach them about "the times" (a sequence of events) and "the seasons" (eras or epochs of time) of these events; the Thessalonians already knew Jesus’ coming will be like a "thief in the night" and a mother going into "labor pains" (1 Thess 5:2-3). Both illustrate difficulty and pain which is sudden, unexpected, and unannounced (Mt 24:42-44; Lk 12:39-40; 2 Pet 3:10; Rev 3:3; 16:15). Neither symbol describes a rapture, but the coming of the Day of the Lord.
The Day of the Lord begins with delusion, people thinking it’s a time of "peace and safety" when it’s really a time of "sudden destruction" (1 Thess 5:3). The "destruction" (olethros) will be "sudden," without warning or "escape."
It’s important to note the change in pronouns from "brethren," "you," and "yourselves" (1 Thess 5:1-2) to "they" and "them" (1 Thess 5:3). The Day of the Lord affects non-believers.

II. The Threat (1 Thess 5:4-7). Truth in the head and heart are useless until it also invades the hands and feet. Truth requires action. Because the "brethren" will not be "overtaken" (katalabe, a hostile capture) by the Day of the Lord, we are not to "sleep as others do" (1 Thess 5:6). The brethren are "sons of light and sons of the day," terms Jesus used to describe saved people (1 Thess 5:5; Lk 16:8; Jn 12:36). Christians are contrasted with unbelievers, whom Paul calls in darkness of the night and asleep.
"Darkness" (skotei) is symbolic of spiritual or moral blindness and separation from the Lord who is light (Col 1:13). Generally, the word is applied to spiritual matters (Jn 8:12; Eph 5:8). Light, on the other hand, pictures purity. Believers and un-believers are opposites in nature. The future Day of the Lord won’t affect God’s children, but our different nature causes a different lifestyle in the present.
Paul uses the word "sleep" (katheudomen) of unbelievers to warn Christians. The Greek word Paul uses here is different, however, from the word he used for the death (koimao) of believers (1 Thess 4:13-15). The word in verse 6 describes an apathetic and indifferent spiritual state. Christians must not be spiritually indifferent but be continually "watchful" (paying attention) and "sober" (calm, self-controlled). The Christian’s very nature in Christ makes him different from the unsaved, so the Christian is to live and think differently from the unsaved.

III. The Tact (1 Thess 5:8-11). The threat of Christian indifference is great enough Paul reminded the Thessalonians to be like ready soldiers. Yet in a one-time act, believers are protected by the core Christian virtues of "faith, hope and love" (1 Thess 1:3; 1 Cor 13:13).
With the Day of the Lord looming, we can "comfort one another" today because we will spend eternity with Jesus. Our salvation is certain and appointed by God through the death of Jesus. This salvation, whether we are dead or alive at Christ’s return, keeps us from God’s wrath in the Day of the Lord, which will be poured out on unbelievers in destruction they cannot escape.