Here, day refers to the day of the Lord. This is a time when God acts in history to save his people and judge his enemies (see Isaiah 13:4–12; Joel 2:1–32; Zephaniah 2:1–3). Christ's first coming began the day of the Lord’s salvation for his people. A future day of the Lord’s judgment on his enemies will occur as well.
the morning star: The planet Venus, called the “morning star,” often appears above the eastern horizon just before dawn. Peter reverses the normal order of nature. He explains that the dawning of the day of the Lord will lead to the Morning Star, Christ, shining in the hearts of believers.