Isaiah 58:5 - Ellicott's Commentary On The Whole Bible

A day for a man to afflict his soul. — The phrase comes from Leviticus 16:29, and describes the soul-sorrow which was the true ideal of fasting. In contrast with this we have the picture, reminding us of Matthew 6:16, of the mechanical prostrations, which are as the waving of a bulrush in the breeze. The image suggests a new aspect of our Lord’s statement, that the Baptist was not as “a reed shaken by the wind” (Matthew 11:7), scil., that his fasting was not outward and ceremonial, like that of the Pharisees.