As the lightning, that lighteneth - See this particularly explained, Matthew 24:27, Matthew 24:28 (note).

For as the lightning that lighteneth out of the one part under heaven,.... The Syriac version reads, "out of heaven", and the Arabic version, "in heaven"; which is the seat of lightning, and from whence it arises:

and shineth unto the other part under heaven; enlightens the earth, which is under the heaven: though the sense of the words, as they lie in the original text, seems to be, that as the lightning lightens at one end of the heavens, and shines to the other; which is done at once, in a moment, in a twinkling of the eye, and to which agrees the Ethiopic version;

so shall also the son of man be in his day: which is not to be understood of the swift progress of the Gospel, after his resurrection and ascension, and the pouring forth of his Spirit; but of his sudden coming, first to take vengeance on the Jewish nation for their rejection of him, and then at the last day, to judge both quick and dead. By his day, is meant his kingdom and glory, or his appearance with power, and great glory: Thus we read (f) for Solomon, "in his hour", that is, in his glory, in the time of his kingdom, when he was in his greatest magnificence.

(f) Misna Bava Metzia, c. 7. sect. 1. & Jarchi & Bartenora in ib. Vid. Shirhashirim Rabba, fol. 15. 4. & 16. 1.

Lighteneth (ἀστράπτουσα)

Only here and Luke 24:4.

For as the lightning, that lighteneth out of the one part under heaven, shineth unto the other part under heaven; so shall also the Son of man be in his day.

17:24 For as the lightning. See notes on Mt 24:16-18. The remainder of the chapter is a condensed and fragmentary report of a part of Matthew, chapter 24.

17:24 So shall also the Son of man be - So swift, so wide, shall his appearing be: In his day - The last day.

24. as lightning . so . the Son of man-that is it will be as manifest. The Lord speaks here of His coming and manifestation in a prophetically indefinite manner, and in these preparatory words blends into one the distinctive epochs [Stier]. When the whole polity of the Jews, civil and ecclesiastical alike, was broken up at once, and its continuance rendered impossible by the destruction of Jerusalem, it became as manifest to all as the lightning of heaven that the kingdom of God had ceased to exist in its old, and had entered on a new and perfectly different form. So it may be again, ere its final and greatest change at the personal coming of Christ, and of which the words in their highest sense are alone true.

17:20-37 The kingdom of God was among the Jews, or rather within some of them. It was a spiritual kingdom, set up in the heart by the power of Divine grace. Observe how it had been with sinners formerly, and in what state the judgments of God, which they had been warned of, found them. Here is shown what a dreadful surprise this destruction will be to the secure and sensual. Thus shall it be in the day when the Son of man is revealed. When Christ came to destroy the Jewish nation by the Roman armies, that nation was found in such a state of false security as is here spoken of. In like manner, when Jesus Christ shall come to judge the world, sinners will be found altogether regardless; for in like manner the sinners of every age go on securely in their evil ways, and remember not their latter end. But wherever the wicked are, who are marked for eternal ruin, they shall be found by the judgments of God.