1 Thessalonians 5:2
Perfectly -- full well.
The Day of the Lord -- Time of judgment and tributional. cf. Joel 1:15, Joel 2:1, Obadiah 1:15, Zephaniah 1:14, Isaiah 2:12, Isaiah 13:6, Ezekiel 13:5. [ 1 Corinthians 1:7-8 ]
"The Day of the Lord" in the prophets had about three elements involved:
1) It would come suddenly and unexpectedly
2) It would involve a great upheaval to the people (kings and rulers falling, as pictured in the Sun and moon ceasing to shine, and mountains upheaved, etc)
3) It would be a time when God would reveal his wrath and bring judgment upon the nations.
While the "Day of Lord" in some prophetic passages may refer to God’s judgment upon Jerusalem, it here refers to his Second Coming, the day of resurrection. John 5:28-29. (Neither does it refer to the day of one’s death.)
5. RETRIBUTION -- 1 Thessalonians 5:1-3
1. Day of the Lord
2. Times and Seasons