ISB ~ 1 Thessalonians 5 v2

day of the Lord:The appointed day when Christ the Lord will come again as Savior (Heb 9:28) and Judge (Mt 25:31-46). Paul does not know when this day will arrive; he knows only some of the signs that will lead up to it (2 Thess 2:1-12). Liturgically, this final and fateful day is anticipated every Lord’s day, when believers gather for eucharistic worship (Rev 1:10) and receive either blessings or curses at Christ’s table (Jn 6:54; 1 Cor 11:27-32). Historically, it is prefigured by the return of Jesus to judge Israel and Jerusalem in A.D. 70.

like a thief:i.e., suddenly and unexpectedly (Mt 24:43-44; Rev 3:3).