Stephen Ricks notes that the only periodic fast prescribed for the Israelites in the Pentateuch (the five books of Moses) occurs on the Day of Atonement, "the tenth day of the seventh month" of the ancient Jewish calendar (the first month of the current Jewish calendar), when the Lord commanded the Israelites to meet together to "afflict [their] souls" (i.e., to fast) and to "offer an offering made by fire unto the Lord" (Leviticus 23:27ff.; see also 16:29, 31; Numbers 29:7-10). There are no references to this festival in the Old Testament outside of the books of Moses, although Isaiah 58:3-7 may refer to abuses of the fast associated with the Day of Atonement.
The observance of the Day of Atonement fast is never explicitly mentioned in the Book of Mormon. However, its observance may be implied in Alma 30:2, where fasting, mourning, and prayer are referred to, and in Alma 45:1, where fasting, rejoicing, and prayer are mentioned as taking place at the beginning of the year (see Alma 44:24), at the time when the Day of Atonement rites were probably observed among the Nephites. This presumption that these two fasts reflect observances associated with the Pentateuch is, I think, strengthened by the observation in Alma 30:3 (following the mention of fasting and associated rites) that "the people did observe to keep the commandments of the Lord; and they were strict in observing the ordinances of God, according to the law of Moses; for they were taught to keep the law of Moses until it should be fulfilled." [Stephen D. Ricks, "Fasting in the Book of Mormon and the Bible" in The Book of Mormon: The Keystone Scripture, p. 128] [See the commentary on Helaman 9:10; Mosiah 27:22-23; Alma 17:3, 9]
Alma 45:1 They did fast much and pray much, and they did worship God with exceeding great joy ([Illustration]): Fasting in the Old Testament. [Stephen D. Ricks, "Fasting in the Book of Mormon and the Bible" in The Book of Mormon: The Keystone Scripture, pp. 134-135]
Alma 45:1 They did fast much and pray much, and they did worship God with exceeding great joy ([Illustration]): Fasting in the Book of Mormon. [Stephen D. Ricks, "Fasting in the Book of Mormon and the Bible" in The Book of Mormon: The Keystone Scripture, p. 136]