The purpose of fasting

Then I turned my face to the LORD God, seeking Him by prayer and plea for mercy with fasting and sackcloth and ashes. -- Daniel 9:3 (ESV)

Believers fast when they "seek God" often with the purpose to urgently plea or to inquire about a case.

Yet they seek Me daily and delight to know my ways, as if they were a nation that did righteousness and did not forsake the judgment of their God; they ask of me righteous judgments; they delight to draw near to God. -- Isaiah 58:2

What is fasting

‘Why have we fasted, and you see it not?
Why have we humbled ourselves, and you take no knowledge of it?’

Behold, in the day of your fast you seek your own pleasure, and oppress all your workers.

Behold, you fast only to quarrel and to fight and to hit with a wicked fist.

Fasting like yours this day will not make your voice to be heard on high.

Is such the fast that I choose, a day for a person to humble himself?
Is it to bow down his head like a reed, and to spread sackcloth and ashes under him? Will you call this a fast, and a day acceptable to the LORD?

“Is not this the fast that I choose:

• to loose the bonds of wickedness,
• to undo the straps of the yoke,
• to let the oppressed go free, and
• to break every yoke?

Is it not to share your bread with the hungry and bring the homeless poor into your house; when you see the naked, to cover him, and not to hide yourself from your own flesh?

-- Isaiah 58:3-7 (ESV)

In other words, fasting is pointless if you are not willing to repent from wickedness, nor "to let the oppressed go free".

This was also the case:

In the fourth year of King Darius, the word of the LORD came to Zechariah on the fourth day of the ninth month, which is Chislev. Now the people of Bethel had sent Sharezer and Regem-melech and their men to entreat the favor of the LORD, saying to the priests of the house of the LORD of hosts and the prophets, “Should I weep and abstain in the fifth month, as I have done for so many years?”

Then the word of the LORD of hosts came to me: “Say to all the people of the land and the priests,

‘When you fasted and mourned in the fifth month and in the seventh, for these seventy years, was it for Me that you fasted?
And when you eat and when you drink, do you not eat for yourselves and drink for yourselves?
Were not these the words that the LORD proclaimed by the former prophets, when Jerusalem was inhabited and prosperous, with her cities around her, and the South and the lowland were inhabited?’ ”

And the word of the Lord came to Zechariah, saying, “Thus says the LORD of hosts,

• Render true judgments,
• show kindness and mercy to one another,
• do not oppress the widow, the fatherless, the sojourner, or the poor, and
• let none of you devise evil against another in your heart.”

-- Zechariah 7:1-10 (ESV)

Jesus advise on fasting:

“And when you fast, do not look gloomy like the hypocrites, for they disfigure their faces that their fasting may be seen by others. Truly, I say to you, they have received their reward. But when you fast, anoint your head and wash your face, that your fasting may not be seen by others but by your Father Who is in secret. And your Father Who sees in secret will reward you. -- Matthew 6:16-18 (ESV)

True fasting means to "humble ourselves before our God, to seek from him" favour in an important matter:

Then I proclaimed a fast there, at the river Ahava, that we might humble ourselves before our God, to seek from him a safe journey for ourselves, our children, and all our goods. For I was ashamed to ask the king for a band of soldiers and horsemen to protect us against the enemy on our way, since we had told the king, “The hand of our God is for good on all who seek him, and the power of his wrath is against all who forsake him.” So we fasted and implored our God for this, and he listened to our entreaty. -- Ezra 8:21-23 (ESV)

For example to abstain from food or drinks:

Then Jesus was led up by the Spirit into the wilderness to be tempted by the devil. And after fasting forty days and forty nights, he was hungry. -- Matthew 4:1-2 (ESV)

Another example in the case of David:

And the LORD afflicted the child that Uriah’s wife bore to David, and he became sick. David therefore sought God on behalf of the child. And David fasted and went in and lay all night on the ground. And the elders of his house stood beside him, to raise him from the ground, but he would not, nor did he eat food with them. -- 2 Samuel 12:15-17 (ESV)

and

When I wept and humbled my soul with fasting, it became my reproach. When I made sackcloth my clothing, I became a byword to them. -- Psalm 69:10-11 (ESV)

and

My knees are weak through fasting; my body has become gaunt, with no fat. -- Psalm 109:24 (ESV)

Another example in the case of Ester:

Then Esther told them to reply to Mordecai, “Go, gather all the Jews to be found in Susa, and hold a fast on my behalf, and do not eat or drink for three days, night or day. I and my young women will also fast as you do. Then I will go to the king, though it is against the law, and if I perish, I perish.” -- Esther 4:16 (ESV)

Often fasting was associated with tearing of clothes and wearing sackcloth (Jonah 3:5), however the LORD Himself says, that he only desires repentance:

“Yet even now,” declares the LORD, “return to me with all your heart, with fasting, with weeping, and with mourning; and rend your hearts and not your garments.” -- Joel 2:12-13 (ESV)

Timing of fasting

It makes no sense to fast in a time that is supposed to be celebrated:

Then the disciples of John came to him, saying, “Why do we and the Pharisees fast, but your disciples do not fast?” And Jesus said to them, "Can the wedding guests mourn as long as the bridegroom is with them? The days will come when the bridegroom is taken away from them, and then they will fast."

-- Matthew 9:14-15 (ESV); Mark 2:18-20; Luke 5:33-35

Benefits of fasting

Then shall your light break forth like the dawn, and

• your healing shall spring up speedily;
• your righteousness shall go before you;
• the glory of the LORD shall be your rear guard.
• Then you shall call, and the LORD will answer; you shall cry, and He will say, ‘Here I am.’

If you take away the yoke from your midst, the pointing of the finger, and speaking wickedness, if you pour yourself out for the hungry and satisfy the desire of the afflicted, then shall your light rise in the darkness and your gloom be as the noonday:

• And the LORD will guide you continually and
• satisfy your desire in scorched places and
• make your bones strong; and
• you shall be like a watered garden, like a spring of water, whose waters do not fail.
• And your ancient ruins shall be rebuilt; you shall raise up the foundations of many generations; you shall be called the repairer of the breach, the restorer of streets to dwell in.

-- Isaiah 58:8-12 (ESV)

Worship

This form of sacrifice is also considered a form of worship:

And there was a prophetess, Anna, the daughter of Phanuel, of the tribe of Asher. She was advanced in years, having lived with her husband seven years from when she was a virgin, and then as a widow until she was eighty-four. She did not depart from the temple, worshiping with fasting and prayer night and day. -- Luke 2:36-37 (ESV)

and

While they were worshiping the Lord and fasting, the Holy Spirit said, “Set apart for me Barnabas and Saul for the work to which I have called them.” Then after fasting and praying they laid their hands on them and sent them off. -- Acts 13:2-3 (ESV)