Paul reminds them of the teachings they already received (3:3–4).
the Day of the Lord: The day when God will judge humanity and save his people (Isaiah 13:6, 9; Ezekiel 30:3; Joel 2:21–32; 3:18; Zechariah 14).
like a thief in the night: Compare Matthew 24:43–44; Luke 12:39–40; 2 Peter 3:10; Revelation 3:3; 16:15.