Charles Spurgeon Commentary Isaiah 58:5-6

"Is such the fast that I have chosen? the day for a man to afflict his soul? Is it to bow down his head as a rush, and to spread sackcloth and ashes under him? wilt thou call this a fast, and an acceptable day to Jehovah? Is not this the fast that I have chosen: to loose the bonds of wickedness, to undo the bands of the yoke, and to let the oppressed go free, and that ye break every yoke?" — Isaiah 58:5-6 (ASV)

Is it such a fast that I have chosen? a day for a man to afflict his soul? is it to bow down his head as a bulrush, and to spread sackcloth and ashes under him? will you call this a fast, and an acceptable day to the LORD? Is not this the fast that I have chosen?

to loose the bands of wickedness,

That is, if by any dishonesty you have got a man in your power, set him free – if you have oppressed him, give him his rights. This is God's kind of fasting.