A lamp shining in dark place is a metaphor that describes the darkness and gloom of the present age. Even in such darkness, the prophetic word is still a lamp that is relevant and illuminates. The day refers to the day of the Lord (2 Peter 3:10) which is a day of salvation (2 Peter 3:12) and judgment (2 Peter 2:9; 2 Peter 3:7)
Throughout Scripture this day is the time of God’s coming to judge humanity and execute wrath (Isaiah 13:6, Isaiah 13:9; Ezekiel 13:5; Ezekiel 30:3; Joel 1:15; Joel 2:1, Joel 2:11; Joel 3:14; Amos 5:18, Amos 5:20; Zephaniah 1:7; Zechariah 14:1; Malachi 4:5; Acts 2:20; 1 Corinthians 5:5; 2 Thessalonians 2:2; 2 Peter 3:10). However, for God’s people this will be a day of salvation (Joel 2:21–32; Joel 3:18; Obadiah 1:15–21; Zechariah 14:1–21).1 Old Testament prophecy is thus still valuable for Christians since it lights up the darkness of this present world’s hopeless ignorance with a bright beam of hope.2
19 And we have the prophetic word more fully confirmed, to which you will do well to pay attention as to a lamp shining in a dark place, until the day dawns and the morning star rises in your hearts,