Until the coming of Christ, the Nephites observed the Law of Moses (2 Nephi 25:24; Alma 25:15), which generally used lunar months (new moon to new moon). The Savior was crucified on the fourteenth day of the first lunar month of the Jewish calendar (John 19:14; Leviticus 233:5), but on the fourth day of the first Nephite month (3 Nephi 8:5). This may imply that Nephite months at that time were not lunar and that their civil calendar may have differed from their religious calendar. [Daniel Ludlow ed., Encyclopedia of Mormonism, p. 170]
“In the Thirty and Fourth Year in the First Month on the Fourth Day of the Month”
The Book of Mormon tells us that Jesus lived very nearly thirty-three Nephite years (3 Nephi 2:8-9; Helaman 14:20; 3 Nephi 8:3-5). According to John Pratt, that Jesus was thirty when he began his ministry is independently verified by the combined witnesses of John and the Book of Mormon. John describes a three year ministry ending at his death at Passover. Thus, the feast day birthday on which he apparently began his ministry was his thirtieth birthday. If so, we can conclude that the Savior fulfilled the symbolism of the law of Moses to the very day.
The Lord told Moses, "I have taken the Levites from among the children of Israel instead of all the firstborn." (Numbers 3:12; see also 8:18) Because the Levites were thus a symbol of Christ as the Firstborn, perhaps we can also expect to learn part of the pattern of the Savior's life from the laws given to the Levites. The law of Moses stated that Levites "thirty years old and upward" should enter into the service of the Lord. (Numbers 4:3,23,30) Consequently, if the law was symbolic of Him, we might expect that Jesus would also begin his ministry at age thirty.
John makes mention of only one miracle that the Savior performed between his baptism and the Passover the following spring. John states that changing water into wine at the marriage in Cana was the "beginning of [the Savior's] miracles." Yet even this miracle was not public. Only the Lord's disciples and the servants knew of it. (see John 2:1-11) Apparently it was not until the Passover, which followed "not many days" later, that Jesus began to perform miracles openly, marking the beginning of his public ministry. Further, John indicates the exact day these public miracles began: "Now when he was in Jerusalem at the passover, in the feast day, many believed in his name, when they saw the miracles which he did." (John 2:23) Thus, appropriately, it appears that Christ began his public ministry on the celebration day of the Passover feast, 15 Nisan. If so, . . . it would not only have been on his birthday, . . . but it would have been near his thirtieth because he was "beginning to be about thirty" at his baptism shortly before (Luke 3:23). [John P. Pratt, "Passover -- Was It Symbolic of His Coming?", The Ensign, January 1994, p. 42]