To fast is to go without food and drink voluntarily for a certain period of time. Fasting combined with sincere prayer can help you prepare yourself and others to receive God’s blessings.
On one occasion, the Savior cast a devil out from a child and used this experience to teach His disciples about the power of prayer and fasting. His disciples asked Him, “Why could not we cast him out?” Jesus answered: “Because of your unbelief: for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you. Howbeit this kind goeth not out but by prayer and fasting.” (See Matthew 17:14–21.)
This account teaches that prayer and fasting can give added strength to those giving and receiving priesthood blessings. The account can also be applied to your personal efforts to live the gospel. If you have a weakness or sin that you have struggled to overcome, you may need to fast and pray in order to receive the help or forgiveness you desire. Like the demon that Christ cast out, your difficulty may be the kind that will go out only through prayer and fasting.
You can fast for many purposes. Fasting is one way of worshiping God and expressing gratitude to Him (see Luke 2:37; Alma 45:1). You can fast as you ask Heavenly Father to bless the sick or afflicted (see Matthew 17:14–21). Fasting may help you and those you love receive personal revelation and become converted to the truth (see Alma 5:46; 6:6). Through fasting you can gain strength to resist temptation (see Isaiah 58:6). You can fast as you strive to humble yourself before God and exercise faith in Jesus Christ (see Omni 1:26; Helaman 3:35). You may fast to receive guidance in sharing the gospel and magnifying Church callings (see Acts 13:2–3; Alma 17:3, 9; 3 Nephi 27:1–2). Fasting may accompany righteous sorrow or mourning (see Alma 28:4–6; 30:1–2).
The Church designates one Sunday each month, usually the first Sunday, as a day of fasting. Proper observance of fast Sunday includes going without food and drink for two consecutive meals, attending fast and testimony meeting, and giving a fast offering to help care for those in need.
Your fast offering should be at least the value of the two meals you do not eat. When possible, be generous and give much more than this amount.
In addition to observing the fast days set aside by Church leaders, you can fast on any other day, according to your needs and the needs of others. However, you should not fast too frequently or for excessive periods of time.
In the Sermon on the Mount, Jesus taught the true form of fasting. He spoke against hypocrites who, when they fast, “disfigure their faces, that they may appear unto men to fast.” Rather than putting on an outward show of righteousness, you should fast “unto thy Father which is in secret: and thy Father, which seeth in secret, shall reward thee openly” (Matthew 6:16–18).
The prophet Isaiah also taught of the true spirit 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? 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?” (Isaiah 58:6–7).
Isaiah also testified of the blessings that come when we obey the law of the fast: “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. Then shalt thou call, and the Lord shall answer; thou shalt cry, and he shall say, Here I am. … 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: 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” (Isaiah 58:8–11).