Albert Barnes 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 – That is, you inhabitants of Jerusalem shall rejoice when the army of the Assyrian is destroyed.

As in the night, when a solemnity is kept – The word ‘solemnity’ here (חג châg) denotes a festival, or feast; and refers, preeminently, to the Passover, which is usually designated as “the feast;” that is, the principal festival of the Jews (John 5:1, 11, 13, 23). This festival was kept at first at night, and was required to be so celebrated from then on (Exodus 12:42; Deuteronomy 16:1–6).

As when one goes with a pipe – Music was used in the daily service of the temple, and their processions and celebrations were all with instrumental music. The simple idea is that the sudden and complete destruction of the army of Sennacherib would be the occasion of the highest joy.