1 Thessalonians 5:3 For when they shall say, Peace and safety; then sudden destruction cometh upon them, as travail upon a woman with child; and they shall not escape.
Paul goes on to write:
1 Thessalonians 5:4 But ye, brethren, are not in darkness, that that day should overtake you as a thief. 5 Ye are all the children of light, and the children of the day: we are not of the night, nor of darkness. 6 Therefore let us not sleep, as do others; but let us watch and be sober.
The implication is that those who are not watching for the return of Christ and living lives that reflect this, will be taken unawares by his return.
Matthew 24:36-51 But of that day and hour knoweth no man, no, not the angels of heaven, but my Father only. 37 But as the days of Noe were, so shall also the coming of the Son of man be. 38 For as in the days that were before the flood they were eating and drinking, marrying and giving in marriage, until the day that Noe entered into the ark, 39 And knew not until the flood came, and took them all away; so shall also the coming of the Son of man be. 40 Then shall two be in the field; the one shall be taken, and the other left. 41 Two women shall be grinding at the mill; the one shall be taken, and the other left. 42 Watch therefore: for ye know not what hour your Lord doth come. 43 But know this, that if the goodman of the house had known in what watch the thief would come, he would have watched, and would not have suffered his house to be broken up. 44 Therefore be ye also ready: for in such an hour as ye think not the Son of man cometh. 45 Who then is a faithful and wise servant, whom his lord hath made ruler over his household, to give them meat in due season? 46 Blessed is that servant, whom his lord when he cometh shall find so doing. 47 Verily I say unto you, That he shall make him ruler over all his goods. 48 But and if that evil servant shall say in his heart, My lord delayeth his coming; 49 And shall begin to smite his fellowservants, and to eat and drink with the drunken; 50 The lord of that servant shall come in a day when he looketh not for him, and in an hour that he is not aware of, 51 And shall cut him asunder, and appoint him his portion with the hypocrites: there shall be weeping and gnashing of teeth.
The point being made is that we should be always ready for Christ’s return because the day and hour cannot be worked out from scripture.
I hope you find this helpful.