What is the “fast” referred to in Isaiah 58:3?
 This must be the fast of the Day of Atonement, the only fast commanded by God (Lev. 16:29, 31; Lev. 23:27-32). This is confirmed in Isaiah 58:3 by the parallel expression “humble ourselves” (NRSV), which follows the terminology of Leviticus. Humbling/afflicting oneself referred to various forms of self-denial, including fasting (compare Ps. 35:13; Dan. 10:2, 3, 12).
 The day of Atonement setting explains God’s command to “Lift up your voice like a trumpet!” (Isa. 58:1, NRSV). This kind of ram’s horn trumpet, called a shofar, was to be blown as a memorial or reminder ten days before the Day of Atonement (Lev. 23:24). Furthermore, every fiftieth year, on the Day of Atonement, it was to announce the beginning of the Jubilee year of freedom (Lev. 25:9, 10; compare Isa. 27:13).
 Read Isaiah 58:3-7. What is the Lord complaining to them about? What was wrong with their “fast”?
 It seems the people were expecting the Lord to congratulate them for their “piety.” Of course, they had it all backward. Practicing self-denial on the Day of Atonement was to express their gratitude and loyalty to Him on the day the high priest went before God to cleanse the sanctuary and thereby cleanse them from sins for which they had already been forgiven (Leviticus 16; compare chapter 4). Their acts should have been done in thankfulness and gratitude to the God who saved them in the day of judgment, not in order to get God’s approval for their “piety” and “devotion.” After all, it was the sins of the people that had defiled God's sanctuary. It had to be cleansed with blood that was shed because of what they had done.