“The day of the Lord Jesus”, referring to Jesus’ coming to assess the saints’ actions while in the flesh, as Paul is using it here, is one day only. It can be understood as one twenty-four (24) hour day that starts at sunset, goes through dawn, the light period, and ends at sunset. It can only be Jesus coming and judging the saints.
“The day of the Lord Jesus” is not plural like “in the last days” used elsewhere. The single day here is Jesus’ second coming, and the plural “in the last days” is the time before his second coming. “The day of the Lord” contains the harvest that Jesus and the apostles taught, and Paul is using here. (Matthew 13:30, 39, 21:34, 41, 25:24, 26; Mark 4:29, 12:2; Luke 20:10; John 4:35-36; Romans 1:13; and 1 Corinthians 9:10-11; Galatians 6:9; Revelation 14:15-16) “The last days”, as Peter stated on Pentecost when he quoted Joel, started with Jesus’ death and resurrection, the day of Pentecost, and will end when Jesus comes on “the day of the Lord”. “The day of the Lord” starts the Lord’s one-thousand-year reign.
Apostle Paul says that he will boast of the Corinthian congregation on the day of the Lord Jesus. When I, like Paul, stand before Jesus to give an account of what I did with my life and the gifts given to me, what can I show to Jesus? What can I boast about? Jesus will say, “What did you do with the talents I gave you?” He will say this to all he calls. “He who is given much, much will be expected.”
