Albert Barnes Commentary Isaiah 12:1

"And in that day thou shalt say, I will give thanks unto thee, O Jehovah; for though thou wast angry with me, thine anger is turned away and thou comfortest me." — Isaiah 12:1 (ASV)

And in that day - This refers to the day mentioned in the previous chapter, the time of the Messiah, when the effects of His reign will be seen everywhere. The duty of praise, however, is expressed in such language that it applies to the event predicted in the former part of the prophecy (Isaiah 10)—the deliverance of the nation from Sennacherib's invasion—as well as to the more glorious event on which the prophet fixed his eye (Isaiah 11)—the coming and reign of the Messiah. The language of this song of praise would be appropriate for both these events.

Thou shalt say - The address to an individual here, using the term ‘thou,’ is equivalent to “everyone,” meaning that all who were thus involved in the divine interposition should say it.

O Lord - O Yahweh—the great author of this interposition.

I will praise thee: though thou wast angry with me - If this language is applied to the Jews, and understood to be used by them concerning the invasion of Sennacherib, it means that God allowed their land to be invaded and subjected to calamities because of their sins (Isaiah 10:6 and following).

If it is understood to apply to the time of the Messiah, then it is language that every redeemed sinner may use: that God was angry with him, but that His anger is turned away.

As it applies to the redeemed, it is an acknowledgment they all feel: that they have no claim to His mercy, and that it lays the foundation for unceasing praise that His anger is turned away by the plan of salvation.