You yourselves know very well (New English Bible, cf. Traduction œcuménique de la Bible “perfectly well”) emphasizes both you and know. At the moment, the point of the comparison with a thief is that he comes as unexpectedly as the Day of the Lord will come (cf. Biblia Dios Habla Hoy “will surprise you”). The idea that the thief comes to do harm is not implied until later. The concept of the Day of the Lord goes back at least to Amos 5.18, 20, where it is already linked with the contrast between light and darkness; but whereas in the Old Testament, “the Lord” was God (that is, Yahweh or Jehovah), for Paul, here as in almost all other passages, “the Lord” is Christ.
The translation should make it clear, first, that the comparison is between two events, the coming of the Lord and the coming of a thief; and secondly, that something which is to happen, and is therefore unknown, is being compared to something which has already happened frequently, and is therefore well known. The addition of the future verb will come indicates clearly that the Day of the Lord is an event in the future. The verb “comes” could be added after thief, since it is implied. Die Bibel im heutigen Deutsch makes the point of comparison explicit by translating “will come as unexpectedly as a thief in the night.”
The image of the thief may be influenced by the saying of Jesus later recorded in Matthew 24.43; Luke 12.39. The same image is used later in 2 Peter 3.10; Revelation 3.3; 16.15. Paul does not need to search for original ways of expressing teaching with which his readers were already well acquainted.
The Day of the Lord will come as a thief at night is far more difficult to translate than one might think. The comparison is between two events, but in the first instance it is “time that comes” and in the second instance it is “the thief that comes.” In many languages it is impossible to speak about “a day coming,” though one can say “a person will come on a certain day.” If, however, one translates “the Lord will come as a thief comes at night,” the comparison may seem to point, not to the unexpectedness of the coming, but to stealthiness or bad intent. Here it may be necessary to say “the Lord will come as unexpectedly as a thief who comes at night” (cf. Phillips), or “the Lord will come when no one expects him, just as a thief comes when no one is expecting him.”
Quoted with permission from Ellingworth, Paul and Nida, Eugene A. A Handbook on Paul’s First Letter to the Thessalonians. (UBS Handbook Series). New York: UBS, 1976. For this and other handbooks for translators see here .