Another interpretation: “This month shall be for you” (Exodus 12:2) – the ways of the Holy One blessed be He is not like the ways of flesh and blood. The way of flesh and blood is that two stand before the king; one prosecutes him [the accused] and one defends him. Not everyone who prosecutes defends, and one who defends does not prosecute. But the Holy One blessed be He is not so; He defends and He prosecutes.
The mouth that said: “Alas, a sinful nation” (Isaiah 1:4), is the mouth that said: “Open the gates, and let the righteous nation enter” (Isaiah 26:2). The mouth that said: “A people laden with iniquity” (Isaiah 1:4), is that which said: “Your people shall all be righteous” (Isaiah 60:21). The mouth that said: “Children who deal corruptly” (Isaiah 1:4), is that which said: “And all your children will be disciples of the Lord” (Isaiah 54:13).
The mouth that said: “Evildoing descendants” (Isaiah 1:4), is that which said: “Their descendants will be renowned among the nations” (Isaiah 61:9). The mouth that said: “Even if you increase your prayers, I will not hear” (Isaiah 1:15), is the mouth that said: “It will be, before they call, I will answer” (Isaiah 65:24). The mouth that said: “Your New Moons and your festivals My soul despises” (Isaiah 1:14), is the mouth that said: “It will be that on every New Moon […all flesh will come to prostrate themselves to Me]” (Isaiah 66:23).
Why does it say: “Your New Moons” [i.e., the celebration of the first of the month]? It is because the months are a gift to Israel, as it is stated: “This month shall be for you.”