The Lord commanded Jeremiah to prophesy to the people “upon the fasting day” (Jer. 36:6), indicating that the Israelites had special days of fasting. To the House of Israel it was one way to show sorrow, especially sorrow for sins, “the children of Israel were assembled with fasting, and with sackclothes, and earth upon them…and stood and confessed their sins” (Nehemiah 9:1-2). Fasting is a way to show humility and contrition before God, “But as for me, when they [enemies] were sick, my clothing was sackcloth: I humbled my soul with fasting; and my prayer returned into mine own bosom” (Psalm 35:13). As an aside, this verse also reveals David’s character – he mourned when his enemies were ill and treated them as he would a friend or brother (see Psalm 35:14). In the book of Joel we find another example of sorrow and fasting, “Therefore also now, saith the LORD, turn ye even to me with all your heart, and with fasting, and with weeping, and with mourning: And rend your heart, and not your garments, and turn unto the LORD your God” (Joel 2:12-13). Is fasting really about mourning and sorrow? It is if you are expressing sorrow for sins or for unfaithfulness to the Bridegroom; however, fasting can also be a thing of great joy.
“And on this [the Lord’s] day thou shalt do none other thing…that thy fasting may be perfect, or in other words, that thy joy may be full. Verily, this is fasting and prayer, or in other words, rejoicing and prayer” (D&C; 59:13-14). This seems quite at odds with how fasting was portrayed in the Old Testament. However, fasting is not only a way to show humility and sorrow but it is also a way of showing devotion to God and personal mastery over the flesh. It is a way to worship God. What could be more joyful than worshiping God?
Fasting is a commandment, “Also, I give unto you a commandment that ye shall continue in prayer and fasting from this time forth” (D&C; 88:76). It’s not something we should take lightly. Another commandment we have received is to read the words of Isaiah [3 Ne. 23:1; see also Mormon 8:23].