Charles Ellicott 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)

You shall have a song ... —The “holy solemnity,” or feast, was probably the Feast of Tabernacles, the feast of in-gathering, and of all the festivals of the Jewish year, the most abounding in joy. In later times, and probably, therefore, in earlier times, it had a night-ritual of special solemnity, the court of the Temple being illuminated with a great candelabrum. It was pre-eminently known as “the feast” (1 Kings 8:2; 1 Kings 8:65; 1 Kings 12:32; Ezekiel 45:25; 2 Chronicles 7:8–9). The second clause of the verse completes the picture by introducing the day-ritual of the procession of pilgrims from the country, bringing their firstfruits and playing on their flutes (Compare to 1 Samuel 10:5).

The mighty One of Israel. —Literally, the Rock of Israel, as a name of Jehovah (Isaiah 17:10; Deuteronomy 32:4, and other passages).