“In that day” — the day of the Lord’s return.
In that day the LORD will start His threshing from the flowing stream of the Euphrates to the brook of Egypt, and you will be gathered up one by one, O sons of Israel. (Isa 27:12)
Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven. Many will say to me in that day, Lord, Lord, have we not prophesied in your name? and in your name have cast out demons? and in your name done many wonderful works? Mtt 7:21-22
Both Old and New testaments use the expression “in that day” to describe the coming day of the Lord. It is a special phrase in the book of Zechariah and Isaiah.
In Zechariah’s 14 chapters it is used 20 times. In the first 31 chapters of the book of Isaiah we have the little phrase “In that day” used 39 times, each which is to be taken as the prophetic Day of the Lord.
There cannot be any doubt about the meaning of “in that Day” in the above-mentioned passages. It is the day of revelation, when persecutors are judged, Christians gain relief from persecution, and marvel at the Lord when they see Him as He is; it is the day of rewards and resurrection; the day of the Glorious Appearing, which the saints love, because it is their blessed hope (Titus 2:13). Alexander Reese
In that day this song will be sung in the land of Judah; “We have a strong city; salvation will God appoint for the walls and bulwarks. Open ye the gates, that the righteous nation which keeps the truth may enter in. For thou wilt keep him in perfect peace, whose mind is stayed on thee: because he trusteth in thee. Trust ye in the LORD for ever: for in the LORD JEHOVAH is everlasting strength.” (Isa 26:1-4)
In what day? In the day that the things are transpiring that he wrote about in chapter 25. And what are the things that he was writing about in chapter 25? What are the days that he was writing about? Verse 8,
“He will swallow up death in victory; and the Lord God will wipe away tears from off all faces; and the rebuke of His people shall He take away from off all the earth: for the Lord hath spoken it. And it shall be said in that day, ‘Lo, this is our God; we have waited for Him, and He will save us: this is the Lord; we have waited for Him, we will be glad and rejoice in His salvation” (Isaiah 25:8-9).
So it is the day that Jesus establishes His kingdom upon the earth. That day for which we’ve been praying when we pray, “Thy kingdom come” (Matthew 6:10). So in that day when His kingdom is established.
(Blue Letter Bible Commentaries – Chuck Smith)