The law of the fast is well explained in Isaiah 58:
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? 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?This description comes in answer to the question why the Lord does not recognize the fast of the Israelites (and smite their enemies), and the answer is that they are fasting wrong.
Our practice is to fast when we seek specific guidance (a newly called president might fast for inspiration in recommending counselors) or specific blessings (as in fasting for the health of someone who is sick, or for one’s own employment). I remember being taught as a child that fasting would be easier if I were fasting for something (when we used to say, only half-joking, that it would be better called "Slow Sunday" instead of Fast Sunday).
I’ll admit it: fasting has never come easily to me, natural man that I am, but I have learned to do it. I really learned to fast on my mission, where – contrary to standard policy, but with permission – our mission president urged us to fast once a week. He often reminded us, however, that the weekly fast was unique for our mission, and that when we returned home, he would recommend we make use of the monthly fast day. He also taught us that it was appropriate to fast for multiple concerns in one fast.
I thought about that teaching as I considered my list of people I was including in my fast yesterday.
I do not pretend to understand the cosmic accounting that measures my faith and the faith of others together with the Lord’s will. But I am grateful for his promise in Isaiah, and for the opportunity to lend my faith in their behalf.