Ye shall have a song - That is, ye 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, by way of eminence, to the Passover, which is usually designated as “the feast;” that is, the principal festival of the Jews (see Matthew 27:15; John 5:1, John 5:11, John 5:13, John 5:23). This festival was kept at first at night, and was required to be so celebrated ever afterward Exodus 12:42; Deuteronomy 16:1-6.
As when one goeth 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.