BUT YOU, BRETHREN, ARE NOT IN DARKNESS, THAT THE DAY SHOULD OVERTAKE YOU LIKE A THIEF: humeis de, adelphoi, ouk este (2PPAI) en skotei, hina he hemera humas hos kleptes katalabe, (3SAAS): (Ro 13:11, 12, 13; Col 1:13; 1P 2:9,10; 1Jn 2:8) (Deuteronomy 19:6; 28:15,45; Je 42:16; Ho 10:9; Zech 1:6)

But you - Placed first in the Greek sentence to provide strong emphasis (Paul does not want his readers to miss the striking contrast in destiny of day people versus night people).  And so Paul dramatically shifts from the third person plural pronoun they (3 times in 1Th 5:3) to the second person plural you. He is now addressing the believers at Thessalonica, emphasizing the day coming like a thief cannot overtake them because they are safe in Christ.

That the day - This should prompt a the 5W & H type question "What day?". To what day or time is Paul referring? Notice even in English, the translation does not say "a day" but "the day". So what? The point is that he is referencing a specific time, not just any day. The Greek construction (using the definite article "he" = "the") before day ("hemera"), substantiates this premise. So again, the day begs the question what day? In this case the context makes this answer quite clear -- Paul is referring to the Day of the Lord, the time he had just mentioned in 1Th 5:2. I believe that the church will be raptured (see comparison of the Rapture vs Second Coming) before the pouring out of God's wrath in the Day of the Lord, and therefore "church age" believers will not be present on earth to experience its terrors and destruction (1Th 5:3). Although most expositors and commentaries state that the day begins just prior to and/or coincides with the Seventieth Week of Daniel. (cp Da 9:27 where "week" = a 7 year period = Daniel's "70th" Week - see notes).

Brethren (80) (adelphos from collative a = copulative prefix {joining together coordinate words} or connective particle serving to join or unite + delphús = womb) is literally one born from same womb and literally identifies a male having the same father and mother. Figuratively (as used throughout this epistle) adelphos refers to a close associate of a group of persons having well-defined membership, specifically identifying fellow believers in Christ united by the bond of affection.

You are (2075) (este) is in the present tense indicating that the believer's continual state is not like that of the unbelieving world - in mental, moral and spiritual darkness because they are still in Adam (1Co 15:22), in sin (Ro 5:12-note, Ep 2:1-note) and in the darkness because they have not believed in the Light (See Jn 1:5; 3:19; 8:12; 2Co 4:4, Ep 4:17, 18-note, speaking to believers Paul says Ep 5:8-note; Ep 5:11-note). As such they are children of Satan (John 8:44) who is called the power of darkness (Lk 22:53).

Not in darkness - Not is (ou) which is the Greek negative meaning absolutely not. It means as believers our position is sure in Christ, the Light of the world. Yes, we can choose to walk in the darkness, for we still have the flesh nature within these decaying physical bodies. The difference is that now that we are children of light, and do not have to stay in the darkness under the dominion of Sin (Jn 8:36, Ro 6:11-note, Ro 6:14-note). How tragic, when a genuine believer chooses to "go back to the leeks, and the onions, and the garlic" of Egypt for a moment, a day or a season (cp Nu 11:5, 6). Dear reader, do not be deceived -- if one habitually, continually abides in the darkness despite a profession of being a child of the Light, their persistence in the darkness proves that they is none of His. Do not be deceived, God is not mocked. Repent. Believe (Mk 1:15, Ac 26:20). Come out from the darkness (cp Is 52:11). Come out from their midst and be separate, for "what fellowship (koinonia - association involving close mutual relations and involvement) has light with darkness"? (2Co 6:14, 15,16, 17, 18)

Spurgeon writing to believers says...

You know that Christ will come (Jn 14:3, 1Th 4:16-note, 1Th 4:17-note). You expect the dissolution of this present state (2Pe 3:10-note, 2Pe 3:11-note). To you therefore, it will come as one who calls at daytime. You cannot know the hour (Mt 24:36). You must not know it; but since you know that He will come (Re 1:7-note), and come to your joy; and since you are in the light (1Jn 1:7, 2:10), you look with gladness to that coming (Titus 2:13-note, Ro 13:11-note, Ro 13:12-note,  2Pe 3:14-note).

Darkness (4655)(skotos [word study] from skia = shadow  thrown by an object, shade caused by the interception of light. Skia it can assume the meaning of skotos and indicate the sphere of darkness) is literally that sphere in which light is absent. In spiritual terms, absence of light by default pictures that which is ensphered in evil and sin.

TDNT notes that skotos...

denotes darkness experienced as an enveloping sphere that has significance for existence, e.g., by hampering movement or foresight, or causing anxiety or danger. If light means potentiality, going into the dark means death. The realm of the dead is a dark realm. It projects already into the present life. Darkness can take the form of blindness. The transferred senses arise as follows. Subjectively, darkness is (1) secrecy or deception, (2) the obscurity of an object or speaker, or (3) lack of knowledge or insight. (Kittel, G., Friedrich, G., & Bromiley, G. W.  Theological Dictionary of the New Testament. Eerdmans)

For He delivered us from the domain of darkness, and transferred us to the kingdom of His beloved Son, (Col 1:13-note)

The day (2250) (hemera) here refers to the specific Day of the Lord.

Overtake (2638) (katalambano from katá = adds intensity [or surprise as in 1Th 5:4] to the meaning of the verb + lambáno = take) means to take eagerly, grasp with force, lay hold of, seize with hostile intent (this literal meaning vividly depicted by the demon who seizes the son and dashed him to the ground in Mark 9:18). Katalambano was used in the sense of laying hold of so as to gain control of. In a secular Greek use we read "they were pursued and overtaken."

Figuratively katalambano is used in the middle voice meaning to "seize" or lay hold of with one's mind and thus to comprehend (Jn 1:5, Ep 3;18) or understand (Ac 4:13, 10:34).

Katalambano in some contexts meant to make something one’s own, to obtain (the prize = win - 1Co 9:24), to attain (Php 3:12, 13, Ro 9:30). In a similar use in the papyri, katalambano is used in the papyri of colonists appropriating land.

NIDNTT gives us the background classic use of the root verb lambano writing  that it...

originally grasp, seize, is attested since Homer. (a) It means to take or grasp. It can indicate both benevolent and hostile actions, and have as object either people or things; e.g. take a wife, collect taxes, accept a verdict, take a road, and fig. take courage. It is used with a material subject, as when, for example, fear or terror seizes men. lambano may serve to enliven the style when used pleonastically. (b) lambano also means to receive, regularly with acc. of the thing; used to embrace all areas of life from simple things to spiritual benefit...  katalambano strengthens the original intention, meaning to seize, take a firm grip, attack; mid. seize for oneself; mental apprehension is basically appropriation and understanding... katalambano is used in the LXX to describe God’s holding grip, and men’s. God’s hand takes hold of the world (Isa. 10:14). He traps men (Job 5:13), and comprehends the incomprehensible (Job 34:24).... The powers of destruction also clutch at man and attack him (Ge 19:19)

Wuest writes that katalambano in Mk 9:18 gives...

The picture in the word is that of seizing something and pulling it down. Our word “katalepsy” comes from the Greek word. Greek writers used it when speaking of fits.

Vine writes that katalambano...

properly means to lay hold of, and then to do this so as to appropriate a thing to oneself, possess as one’s own (cp. 1Co 9:24, “attain”). (Vine, W. Collected writings of W. E. Vine. Nashville: Thomas Nelson or Logos)

Katalambano was used in secular Greek of evils seizing with hostile intent, coming upon or overtaking someone which is somewhat of the sense Paul uses katalambano to describe the Day of the Lord "seizing upon" or coming suddenly upon unbelievers in the same way a thief would come into one's house and seize our possessions! The Greeks also used katalambano of of night, evening or darkness coming upon a person. Jesus uses katalambano with this sense (speaking of spiritual darkness) declaring to the multitudes...

For a little while longer the light (He is alluding to Himself, Jn 1:9, 8:12) is among you. Walk (present imperative - as your lifestyle, your habitual practice) while you have the light, that darkness (skotia - absence of light, personified by John of the forces hostile to God) may not overtake (katalambano - grasp or seize) you; he who walks (present tense = as the habit of their life, as manifest by their lifestyle) in the darkness (skotia) does not know where he goes. (John 12:35)

Here are all 15 uses of katalambano...[attained(1), caught(2), comprehend (2), found(1), laid hold of(2), lay hold of(1),overtake(2), seizes(1), understand(1), understood(1), win(1)]

Mark 9:18 and whenever it seizes him, it dashes him to the ground and he foams at the mouth, and grinds his teeth, and stiffens out. And I told Your disciples to cast it out, and they could not do it."

John 1:5 And the light shines in the darkness, and the darkness did not comprehend it.

Comment: Here katalambano can mean either either "comprehend" mentally, or "seize hold of" with hostile intent.

Amplified = [put it out or absorbed it or appropriated it, and is unreceptive to it].

ASV = apprehended it not

BBE = not overcome by the dark

Darby = apprehended it not

ESV = apprehended it not

GNT = has never put it out

ISV = has never put it out

KJV = the darkness comprehended it not

NAB = has not overcome it

NET =  has not mastered it

NIV = has not understood it

NJB =  has not mastered it

NLT = can never extinguish it

Phillips = has never put it out

Weymouth (and NCV) =  has not overpowered it

Wuest = did not overwhelm it

John 8:3 And the scribes and the Pharisees brought a woman caught in adultery, and having set her in the midst, 4 they said to Him, "Teacher, this woman has been caught in adultery, in the very act.

John 12:35 (see above)

Acts 4:13 Now as they observed the confidence of Peter and John, and understood (discovered = NET, perceived = ESV, KJV, YLT that they were uneducated and untrained men, they were marveling, and began to recognize them as having been with Jesus.

Acts 10:34 And opening his mouth, Peter said: "I most certainly understand (Idea = to seize with the mind) now that God is not one to show partiality,

Acts 25:25 "But I found that he had committed nothing worthy of death; and since he himself appealed to the Emperor, I decided to send him.

Romans 9:30  What shall we say then? That Gentiles, who did not pursue righteousness, attained righteousness, even the righteousness which is by faith;

1 Corinthians 9:24  Do you not know that those who run in a race all run, but only one receives the prize? Run in such a way that you may win (obtain it = ESV, YLT, make it yours = Amplified).

Ephesians 3:18 may be able to comprehend with all the saints what is the breadth and length and height and depth

Comment: Wuest writes that "The word “comprehend” conveys to the English reader the idea of “understand.” The Greek word means “to lay hold of so as to make one’s own, to seize, take possession of.” One could translate “apprehend,” in the sense of mentally grasping some idea or truth.

Php 3:12  Not that I have already obtained it, or have already become perfect, but I press on in order that I may lay hold of  that for which also I was laid hold of by Christ Jesus. 13 Brethren, I do not regard myself as having laid hold of (Friberg says that "with kata adding certainty to possession") it yet; but one thing I do: forgetting what lies behind and reaching forward to what lies ahead,

1Th 5:4 But you, brethren, are not in darkness, that the day should overtake (Friberg says that "with kata adding suddenness") you like a thief

Katalambano is used 120 times in the non-Apocryphal Septuagint (LXX) Ge 19:19; 31:23, 25; 44:4; Ex 15:9; 22:4; Lv 26:5; Nu 21:32; 32:23; Dt. 19:6; 28:15, 45; Jos. 2:5; 8:19; 10:19; 11:10; 19:47; Jdg. 1:5,6, 8; 7:24; 9:45, 50; 18:22; 1Sa 30:8; 2Sa 5:7; 12:26,27, 29; 15:14; 21:11; 1Ki. 18:44; 2Ki. 18:10; 25:5; 2Chr 9:20; 22:9; 25:23; 33:11; Neh 9:25; Esther 4:17; Job 5:13; 34:24; Ps 7:5; 18:37; 40:12; 69:24; 71:11; Pr 1:13; 2:17, 19; 11:27; 13:21; 16:32; Is 10:14; 20:1; 35:10; 37:8; 51:11; 59:9; Je 3:8; 10:19; 42:16; 51:34; 52:8; Lam 1:3; Eze 33:4; Da 1:20; 4:1; 6:11; Ho 2:7; 10:9; Amos 9:13; Obad 1:6; Mic 6:6; Zec 1:6

Genesis 19:19 (Context = similar to the Day of the Lord for it was in the day when God judged the wickedness of Sodom and Gomorrah) "Now behold, your servant has found favor in your sight, and you have magnified your lovingkindness, which you have shown me by saving my life; but I cannot escape to the mountains, lest the disaster overtake (Heb = dabaq = cleave, stick like glue; Lxx = katalambano) me and I die;
 

Psalm 7:5 Let the enemy pursue my soul and overtake (Lxx = katalambano) it; And let him trample my life down to the ground, And lay my glory in the dust. Selah.

Psalm 18:37 I pursued my enemies and overtook (Lxx = katalambano) them, And I did not turn back until they were consumed.

Psalm 40:12 For evils beyond number have surrounded me; My iniquities have overtaken (Lxx = katalambano) me, so that I am not able to see; They are more numerous than the hairs of my head; And my heart has failed me.

Psalm 69:24 Pour out Thine indignation on them, And may Thy burning anger overtake (Lxx = katalambano) them.

Thief (2812)(kleptes from klépto = steal; kleptomaniac) is a stealer or thief who acts with stealth or subterfuge.

Kleptes contrasts with lestes (3027), Jesus using both terms in His discussion of sheep and security in John 10...

Truly, truly, I say to you, he who does not enter by the door into the fold of the sheep, but climbs up some other way, he is a thief (kleptes) and a robber (lestes)...All who came before Me are thieves (kleptes) and robbers (lestes), but the sheep did not hear them. (Jn 10:1,8)

The kleptes steals by fraud and in secret (Mt 24:43; Jn 12:6) whereas the robber or lestes steals by violence and openly. The NT uses kleptes in a figurative sense to describe the false teachers and deceivers who "steal" men away from the truth. In the present context kleptes is used as a figure of speech ("like a thief" - see term of comparison = simile) to describe the sudden and unexpected appearance of the Day of the Lord.

Kleptes - 16x in 16v in the NAS - Mt 6:19, 20-note; 24:43; Lk. 12:33, 39; Jn. 10:1, 8, 10; 12:6; 1Co. 6:10; 1Th. 5:2, 4; 1Pe 4:15-note; 2Pe 3:10-note; Re 3:3-note; Re 16:15-note

The concept of classifying people spiritually as day people or night people is not new to Paul but was seen in the OT as exemplified by this passage from the Psalms...

Psalm 107:10 There were those (referring His chosen people Israel) who dwelt in darkness and in the shadow of death, prisoners in misery and chains (Why were they in darkness?) 11 Because they had rebelled against the words of God, And spurned the counsel of the Most High.

And so here in Thessalonians we get a picture of the terrible bondage of being in the night and in the darkness. As an aside we should note that it is never God's desire that men and women stay in the bondage of spiritual darkness but to be set free, a desire that reverberates through the following passages...

The people who walk in darkness (here refers to Gentiles - see Mt 4:14-16) will see a great light (an allusion to the advent of the Messiah) Those who live in a dark land, The light will shine on them. (Isaiah 9:2)

Again therefore Jesus spoke to them, saying, "I am the light of the world; he who follows Me shall not walk in the darkness, but shall have the light of life." (John 8:12)

(Jesus sent Paul to speak the Gospel to the Gentiles) to open their eyes so that they may turn from darkness to light and from the dominion of Satan to God, in order that they may receive forgiveness of sins and an inheritance among those who have been sanctified by faith in Me.' (Acts 26:18)