Sackcloth and Ashes
In the Bible, when there was cause for sadness and mourning a man would "rent his mantle," put on sackcloth and stand, sit, or lie in ashes to show humility and sadness. The mantle was like a scarf that indicated the authority of a prophet or a king. Sackcloth was (just as it sounds) like a burlap grain sack. The mantle was torn to show disapproval, sadness, and mourning. The man would then put on sackcloth and stand or sit in ashes to demonstrate his humility, sadness and grief. Many times an outward show of humility also included fasting.
God was not always pleased with this outward show of humility and sadness. While humility to God and His Word is always right and appropriate, an outward show is not. God would have us act in a way that meets the needs of others and demonstrates our joy and gladness in trusting Him to take care of the situation causing our grief. God has promised that He will turn our mourning into dancing and He will cloth us with gladness.
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? wilt thou 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, to undo the heavy burdens, and to let the oppressed go free, and that ye break every yoke? - Isaiah 58:5-6