The Gospels often connect the coming of Christ with nighttime (Matt. 24:43; 25:6; Mark 13:35; Luke 12:35-38; 17:34). The early church took the illustrations literally and expected the Lord’s return to occur around midnight. Jerome mentions a Jewish tradition, based on the midnight coming of the Lord at the first Passover in Egypt, that the Messiah was to come at midnight. He attributes to this the “apostolic tradition” that the pre-Easter vigil should continue until midnight, in anticipation of the coming of Christ (Jerome, on Matt. 25:6). The apostle’s use of the word “night” should be seen, also, in the context of vs. 4-6.

 Compare Christ’s words in Matt. 24:43; Luke 12:39, 40. Compare also 2 Peter 3:10; Rev. 3:3; 16:15. By his use of the figure of a thief the apostle emphasizes the unexpectedness of the second coming, thus warning his readers to be ready for its occurring at any time (see on 1 Thess. 4:15). If they heed his words, they need not be taken unawares (cf. Luke 21:34-36; 1 Thess. 5:4).

The form of the verb in Greek stresses the certainty of the Lord’s return.

 Important textual evidence may be cited (cf. p. 10) for the omission of the first “the.” Such an omission in Greek possibly indicates that the phrase “day of the Lord” was a formula understood by the church. For the meaning of the phrase see on Acts 2:20. Paul makes frequent reference to the formula, or its equivalent, sometimes abbreviating it as “the day,” or “that day” (1 Thess. 5:4; Rom. 2:16; 1 Cor. 1:8; 5:5; 2 Cor. 1:14; 2 Thess. 1:10; 2:2). Here, “the day of the Lord” refers to Christ’s second advent.

 Rather, “know accurately.” Not that the Thessalonians knew all there was to know about “the day of the Lord,” but that they were well aware of its sudden coming. From this it is clear the apostle had given them the Lord’s teaching on the subject (Matt. 24:32-44; etc.). There was need only to confirm them in their knowledge and to press home its significance.