Isaiah 58

Herein, the Lord chastises Israel because all they do is according to the letter of the law, but their hearts and desires are far from God.  It is those that seem to display outward worship but do not do so in private or in their hearts.  Of such the Lord describes -

"...they ask of me the ordinances of justice; they take delight in approaching to God."  (Old Testament | Isaiah 58:2)

But are knowingly are not fully committed but yet want to demand and debate that God should bless them.

The Lord chastises -

"Wherefore have we fasted, say they, and thou seest not? wherefore have we afflicted our soul, and thou takest no knowledge? Behold, in the day of your fast ye find pleasure, and exact all your labours.
4 Behold, ye fast for strife and debate, and to smite with the fist of wickedness..."  (Old Testament | Isaiah 58:3 - 4)

To such the Lord  asks as if to teach -

"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?"  (Old Testament | Isaiah 58:5)

The Lord then teaches not only the letter but the spirit and intention of the law of the fast -

"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?
7 Is it not to deal thy bread to the hungry, and that thou bring the poor that are cast out to thy house? when thou seest the naked, that thou cover him; and that thou hide not thyself from thine own flesh?"  (Old Testament | Isaiah 58:6 - 7)

This is the purpose or the fast.  But with every commandment comes great blessings for as we properly fast and bless the poor and the needy -

"Then shall thy light break forth as the morning, and thine health shall spring forth speedily: and thy righteousness shall go before thee; the glory of the LORD shall be thy rereward.
9 Then shalt thou call, and the LORD shall answer; thou shalt cry, and he shall say, Here I am. If thou take away from the midst of thee the yoke, the putting forth of the finger, and speaking vanity;
10 And if thou draw out thy soul to the hungry, and satisfy the afflicted soul; then shall thy light rise in obscurity, and thy darkness be as the noonday:
11 And the LORD shall guide thee continually, and satisfy thy soul in drought, and make fat thy bones: and thou shalt be like a watered garden, and like a spring of water, whose waters fail not.
12 And they that shall be of thee shall build the old waste places: thou shalt raise up the foundations of many generations; and thou shalt be called, The repairer of the breach, The restorer of paths to dwell in."  (Old Testament | Isaiah 58:8 - 12)

This not only seems as a significant law of health but a promise to one's posterity.  The same is had of the Sabbath day -

"If thou turn away thy foot from the sabbath, from doing thy pleasure on my holy day; and call the sabbath a delight, the holy of the LORD, honourable; and shalt honour him, not doing thine own ways, nor finding thine own pleasure, nor speaking thine own words:
14 Then shalt thou delight thyself in the LORD; and I will cause thee to ride upon the high places of the earth, and feed thee with the heritage of Jacob thy father: for the mouth of the LORD hath spoken it."  (Old Testament | Isaiah 58:13 - 14)

Obviously, great blessings come as we come to understand the purpose of his commandments and obey in the spirit of his laws.