For yourselves know perfectly that the day of the Lord so cometh as a thief in the night.

for ye know perfectly well yourselves, that the day of the Lord so comes as a thief by night.

for yourselves have known thoroughly that the day of the Lord as a thief in the night doth so come,

For you yourselves have the knowledge that the day of the Lord will come like a thief in the night.

For you yourselves thoroughly understand that the day of the Lord shall arrive much like a thief in the night.

Ye know perfectly. He places exact knowledge in contrast with an anxious desire of investigation. But what is it that he says the Thessalonians know accurately? [1] It is, that the day of Christ will come suddenly and unexpectedly, so as to take unbelievers by surprise, as a thief does those that are asleep. This, however, is opposed to evident tokens, which might portend afar off his coming to the world. Hence it were foolish to wish to determine the time precisely from presages or prodigies.

Footnotes

1 - "Plenement et certainement;" -- "Fully and certainly."

For yourselves know perfectly - That is, they had been fully taught this. There could be no doubt in their minds respecting it.
The day of the Lord so cometh - Of the Lord Jesus - for so the word "Lord" in the New Testament commonly means; see the notes, Acts 1:24. The "day of the Lord" means that day in which he will be manifested, or in which he will be the prominent object in view of the assembled universe.
As a thief in the night - Suddenly and unexpectedly, as a robber breaks into a dwelling. A thief comes without giving any warning, or any indications of his approach. He not only gives none, but he is careful that none shall be given. It is a point with him that, if possible, the man whose house he is about to rob shall have no means of ascertaining his approach until he comes suddenly upon him; compare Matthew 24:37-43 notes; Luke 12:39-40 notes. In this way the Lord Jesus will return to judgment; and this proves that all the attempts to determine the day, the year, or the century when he will come, must be fallacious. He intends that his coming to this world shall be sudden and unexpected, "like that of a thief in the night;" that there shall be no such indications of his approach that it shall not be sudden and unexpected; and that no warning of it shall be given so that people may know the time of his appearing. If this be not the point of the comparison in expressions like this, what is it? Is there anything else in which his coming will resemble that of a thief? And if this be the true point of comparison, how can it be true that people can ascertain when that is to occur? Assuredly, if they can, his coming will not be like that of a thief; comp. notes on Acts 1:7.

For yourselves know perfectly,.... With great exactness and accuracy, with great clearness and perspicuity, as a certain truth, which was made plain and evident to them, and about which there could be no question; and which perfect knowledge they had, either from the words of Christ, Matthew 24:42, or from the ministration of the apostle and his fellow labourers, when among them:
that the day of the Lord; of the Lord Jesus, when he will show himself to be King of kings, and Lord of lords, and the Judge of the whole earth; and which is sometimes styled the day of the Son of man, and the day of God, for Christ will appear then most gloriously, both in his divine and human nature; the day of redemption, that is, of the body from the grave, and from corruption and mortality; and the last day in which will be the resurrection of the dead, and the day of judgment, in which Christ will come to judge the quick and dead: and which
so cometh as a thief in the night; at an unawares, and the Lord himself in that day will so come, Revelation 3:3 respect is had not to the character of the thief, nor to the end of his coming; but to the manner of it, in the dark, indiscernibly, suddenly, and when not thought of and looked for; and such will be the coming of Christ, it will be sudden, and unknown before hand, and when least thought of and expected: and since the Thessalonians knew this full well, it was needless for the apostle to write about the time and season of it; which they were sensible of, could no more be known and fixed, than the coming of a thief into anyone of their houses.

as a thief in the night--The apostles in this image follow the parable of their Lord, expressing how the Lord's coming shall take men by surprise (Matthew 24:43; 2-Peter 3:10). "The night is wherever there is quiet unconcern" [BENGEL]. "At midnight" (perhaps figurative: to some parts of the earth it will be literal night), Matthew 25:6. The thief not only gives no notice of his approach but takes all precaution to prevent the household knowing of it. So the Lord (Revelation 16:15). Signs will precede the coming, to confirm the patient hope of the watchful believer; but the coming itself shall be sudden at last (Matthew 24:32-36; Luke 21:25-32, Luke 21:35).

For this in general ye do know; and ye can and need know no more.