Before the gospel is preached to all the cities of Israel.

"Ye shall not have gone over the cities of Israel, till the Son of man be come." Matthew 10:23

After the gospel is preached to all the nations of the earth.

"And this gospel of the kingdom shall be preached in all the world for a witness unto all nations; and then shall the end come." Matthew 24:14

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Matthew 10:23 denotes that the Son of man’s(Jesus’) return is actually Him showing Himself to His disciples after His resurrection. Jesus also showed Himself in a glorified state to the Apostle John in the Revelation. There is nothing in verse 10:23 which suggests that it is referring to the End Times; it is just describing the mission of the then present disciples, whom He was sending out to the cities for the first time.

"And Jacob called unto his sons, and said, Gather yourselves together, that I may tell you that which shall befall you in the last days... Dan shall judge his people, as one of the tribes of Israel. Dan shall be a serpent by the way, an adder in the path, that biteth the horse heels, so that his rider shall fall backward. I have waited for thy salvation, O LORD." Genesis 49: 1, 16-18