the Bible say about fasting

Fasting means self-denial by going without food for a period of time. Fasting may be total or partial -- avoiding certain foods or eating smaller than normal quantities. The origin of fasting as a religious practice is unclear, but both the Old and New Testaments of the Bible mention a number of instances of fasting for various reasons:

Distress and Grief. Loss of appetite is a natural reaction in times of distress, grief and mourning, and fasting was considered appropriate at these times. David fasted as a sign of grief when Abner was murdered (2 Samuel 3:35). There was a seven-day fast at the death of Saul (1 Samuel 31:13).

Spiritual Preparation. Fasting is a self-sacrifice that makes one humble and more accepting of God's will. Moses fasted for forty days in preparation for receiving the Ten Commandments (Exodus 34:28). Daniel fasted for three weeks before receiving his vision (Daniel 10:2-6). Elijah fasted forty days before speaking with God (1 Kings 19:8). Jesus fasted for forty days in preparation for His temptation by the devil (Matthew 4:1-11, Luke 4:1-13).

Repentance and Atonement. When Jonah predicted the downfall of Nineveh, The Ninevites fasted as a sign of repentance in hopes God would spare their city (Jonah 3:3-9). The Day of Atonement was an annual obligatory day of rest and fasting for the Israelites (Numbers 29:7). When the Israelites had sinned, they often humbled themselves and fasted in hopes of regaining God's favor (Judges 20:26, 1 Samuel 7:6).

In both the Old and New Testaments, fasting is seen as useful for humbling oneself as a sign of commitment or repentance and for increasing faith, especially when accompanied by prayer. Fasting allowed one to be devoted to spiritual matters without distraction from earthly things. However, fasting was not to be considered an end in itself, nor a substitute for obedience to God and doing good deeds (Isaiah 58:3-10).

Jesus said that fasting, like prayer, should be done in private and not for show (Matthew 6:16-18, cf., Matthew 6:5-7). John the Baptist's disciples routinely fasted according to Jewish custom, but Jesus and His disciples did not. However, Jesus said His disciples would mourn and fast after He had left them (Matthew 9:14-15; Mark 2:18-20; Luke 5:33-35). The early Christians practiced fasting at least occasionally (Acts 13:3, 14:23, 2 Corinthians 6:5, 11:27).

Despite the tradition of fasting in the Bible, and Jesus' references to it, the New Testament teachings do not require fasting, and neither Jesus nor His disciples made fasting obligatory. However, a tradition of partial fasting on Wednesdays, and especially on Fridays dates back to the early days of Christianity.

Church teachings about fasting vary. Many Catholics observe partial fasting traditions during Lent (the period between Ash Wednesday and Easter). Orthodox Christians observe even more fasting days. Most Protestant churches do not have any firm rules or traditions about fasting.