Isaiah 58:5

“Is it such a fast that I have chosen?

The day for a man to afflict his person?

Is it to bow down his head as a rush?

And to spread sackcloth and ashes under him?

Will you call this a fast?

And an acceptable day to Yahweh?”

Yahweh denounces the kind of fasting they were engaging in. (It bears all the marks of the way the worst of the Pharisees fasted (Matthew 6:16)). They afflicted their persons, they bowed their heads in false humility like a rush bows before the wind, they spread sackcloth and ashes underneath them, they wanted all to see that they were fasting. And they meanwhile engaged in their unjust and sinful behaviour. But such a day of fasting is not acceptable to Yahweh. It is a travesty of the ‘acceptable year' of God's deliverance (Isaiah 61:2).