5:2 day of the Lord. This is the first written mention of “the day of the Lord” in the New Testament, although Peter had quoted the phrase from the Old Testament (Joel 2:31) in his sermon on the day of Pentecost (Acts 2:20). This phrase is used over thirty times in the Old Testament. In context here, it refers to the coming of the final period of God’s judgment on the earth.
5:2 thief. Although the day of the Lord would be a period of great judgment on God’s enemies, it would begin unexpectedly and quietly. The Lord Jesus had also used the figure of the thief coming secretly, and the Thessalonians already knew this from Paul’s teachings (see Matthew 24:42-44; also note II Peter 3:10; Revelation 3:3; 16:15). Therefore, we should be constantly watchful for the Lord, not for various signs preceding His coming. Paul was watching throughout his life (II Timothy 4:8), and here he urges: “let us watch and be sober” (I Thessalonians 5:6).