https://www.gotquestions.org/sackcloth-and-ashes.html
“Sackcloth and ashes were used in Old Testament times as a symbol of debasement, mourning, and/or repentance. Someone wanting to show his repentant heart would often wear sackcloth, sit in ashes, and put ashes on top of his head. Sackcloth was a coarse material usually made of black goat’s hair, making it quite uncomfortable to wear. The ashes signified desolation and ruin.
When someone died, the act of putting on sackcloth showed heartfelt sorrow for the loss of that person. We see an example of this when David mourned the death of Abner, the commander of Saul’s army (2 Samuel 3:31). Jacob also demonstrated his grief by wearing sackcloth when he thought his son Joseph had been killed (Genesis 37:34). These instances of mourning for the dead mention sackcloth but not ashes.
Ashes accompanied sackcloth in times of national disaster or repenting from sin. Esther 4:1, for instance, describes Mordecai tearing his clothes, putting on sackcloth and ashes, and walking out into the city “wailing loudly and bitterly.” This was Mordecai’s reaction to King Xerxes’ declaration giving the wicked Haman authority to destroy the Jews (see Esther 3:8–15). Mordecai was not the only one who grieved. “In every province to which the edict and order of the king came, there was great mourning among the Jews, with fasting, weeping, and wailing. Many lay in sackcloth and ashes” (Esther 4:3). The Jews responded to the devastating news concerning their race with sackcloth and ashes, showing their intense grief and distress.
Sackcloth and ashes were also used as a public sign of repentance and humility before God. When Jonah declared to the people of Nineveh that God was going to destroy them for their wickedness, everyone from the king on down responded with repentance, fasting, and sackcloth and ashes (Jonah 3:5–7). They even put sackcloth on their animals (verse 8). Their reasoning was, “Who knows? God may yet relent and with compassion turn from his fierce anger so that we will not perish” (verse 9). This is interesting because the Bible never says that Jonah’s message included any mention of God’s mercy; but mercy is what they received. It’s clear that the Ninevites’ donning of sackcloth and ashes was not a meaningless show. God saw genuine change—a humble change of heart represented by the sackcloth and ashes—and it caused Him to “relent” and not bring about His plan to destroy them (Jonah 3:10).
Other people the Bible mentions wearing sackcloth include King Hezekiah (Isaiah 37:1), Eliakim (2 Kings 19:2), King Ahab (1 Kings 21:27), the elders of Jerusalem (Lamentations 2:10), Daniel (Daniel 9:3), and the two witness in Revelation 11:3.
Very simply, sackcloth and ashes were used as an outward sign of one’s inward condition. Such a symbol made one’s change of heart visible and demonstrated the sincerity of one’s grief and/or repentance. It was not the act of putting on sackcloth and ashes itself that moved God to intervene, but the humility that such an action demonstrated (see 1 Samuel 16:7). God’s forgiveness in response to genuine repentance is celebrated by David’s words: “You removed my sackcloth and clothed me with joy” (Psalm 30:11).”
Love & His mercy,
Matthew