In Jarom 1:5 we find that "the people of Nephi had waxed strong in the land. They observed to keep the law of Moses and the sabbath day holy unto the Lord. . . . And the laws of the land were exceedingly strict."
According to John Welch, while Jarom may have had in mind only the weekly sabbath, he may also have been speaking of the holy days such as Passover, Pentecost, and the Day of Atonement, for those days were also holy days under the law of Moses. For example, assuming that a version of Leviticus 16 was found on the plates of brass, then the Nephites celebrated the Day of Atonement with its respective temple ordinances, for the law defined that day as "a sabbath of rest unto you" (Leviticus 16:31). The Day of Atonement was a sabbath no matter on what day of the week it fell. Although we cannot know for sure which holy days were considered sabbaths by Lehi or his posterity or how they observed them, Jarom's statement puts us on notice that the Nephites were strict in some way to observe each day that was a sabbath under their law, which most likely would have required the observance of certain temple-related holy days. [John W. Welch, "The Temple in the Book of Mormon," in Temples of the Ancient World, pp. 303-304]
“Keep the Law of Moses and the Sabbath Day Holy Unto the Lord”
Susan Easton Black notes that for the ancient Israelites, only the seventh day of the week was assigned a name--Sabbath. Sabbath is derived from the Hebrew word shabat, meaning to break off, to desist, and to rest. On this holy day, "the last in creation, [but] first in mention," the children of Israel remembered and observed the goodness of God.
The Sabbath remains a sacred sign (Hebrew: ot) for the Jews. As the rainbow is a sign that the flood waters will never cover the earth again and as circumcision is a sign of the Abrahamic covenant, the Sabbath is the sign of the sanctifying covenant between God and Israel. Exodus 31:13 states, "Verily my Sabbaths ye shall keep: for it is a sign [ot] between me and you throughout your generations; that ye may know that I am the Lord that doth sanctify you." The invitation to the covenant of Sabbath is recorded in the Decalogue, "Remember the Sabbath day, to keep it holy" (Exodus 20:8). According to the biblical account, God's command to keep the Sabbath holy was first given in conjunction with gathering a double portion of manna on the sixth day to preserve the sanctity of the Sabbath (Exodus 16:5, 22-30). This coincides with a major theme of the Sabbath, the divine process of creation wherein God rested the seventh day: "wherefore the Lord blessed the Sabbath day, and hallowed it" (Exodus 20:11). [Susan Easton Black, "The Sabbath As a Covenant in Mormonism and Judaism," in Covenant and Chosenness in Judaism and Mormonism, pp. 59-60]