John Calvin Commentary Isaiah 30:29

"Ye shall have a song as in the night when a holy feast is kept; and gladness of heart, as when one goeth with a pipe to come unto the mountain of Jehovah, to the Rock of Israel." — Isaiah 30:29 (ASV)

And you shall have a song as in the night. Here he declares that all the punishments which he threatened against the Assyrians will tend to the advantage of the Church, because the Lord punishes the outrages committed against His people no less severely than if they had been committed against Himself. In this way He testifies to His infinite love and kindness toward His own people, when He deigns to take up arms on their behalf. Therefore, we should conclude that all the threats which are found in any part of Scripture tend to the consolation of believers.

When a festival is kept. He says that this “song” will be sacred, and compares it to a “holy solemnity,” in order to excite believers to thankfulness, and to show that their joy should be directed to God; for it is not enough to rejoice unless our joy is directed straight toward God, and unless we keep Him alone always in our view; otherwise, our joy will be fruitless and irreligious, and will not promote our salvation or be acceptable to God. He calls it “a song of the night,” because the Jews began the day at sunset, and, as soon as evening came, celebrated the festival.

To the mountain. He explains more fully what the nature of this joy will be. They will not dance, as irreligious men do, but will raise and fix their eyes on God, whom they acknowledge to be the author of every blessing. By “the mountain” he means the temple that was built “on the mountain.” He calls God The Mighty One of Israel, because it was by His assistance that they had been redeemed and preserved; and therefore he reminds them that in the future they will not be safe in any other way than by placing their hope in God alone. And indeed, when we cherish any belief in our own strength, we rob God of this title, which is truly and sincerely bestowed on Him only by the lowly and humble, who have laid aside all confidence in their own strength.