According to President Brigham Young, the observance of it was originally made necessary for the taking care of the poor. They came, he says, to the Prophet Joseph Smith in Kirtland, before the Law of Tithing had been revealed, and from him wanted assistance. It was decided upon setting aside one day monthly for the poor. The intention was to carry all the flour, meat, butter, fruit, etc., that ordinarily would be eaten on that day to someone selected to distribute it among the needy Saints.
Among the Jews, fasting became a frequent observance. It was considered an expression of penitence, in connection with prayer, or as a sacrifice and a sign of mourning, or great piety. The Lord condemns all ostentation on such occasions as hypocrisy. He says, "When thou fasteth, anoint your head and wash thy face." That is, make it an occasion of joy and gladness. Do not disfigure your faces with dust and ashes, as the hypocrites do. Then, "thy Father, Who seeth in secret, shall reward thee openly."
The Prophet Isaiah, with whose writings the Nephites were familiar, says, "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 it not to deal thy bread to the hungry, and that thou bring the poor that are cast out, to thine house? When thou seest the naked, that thou cover him; and that thou hide not thyself from thine own flesh. (Isaiah 58:5-7)