According to an article by Welch, Parry, and Ricks, the phrase “this day” (Mosiah 2:9) may be very significant in the scriptures. This solemn and emphatic concept appears, for example, in the famous covenantal text at the end of the book of Joshua: “Choose you this day whom ye will serve … ” (Joshua 24:15-25).
The words “this day” appear eighteen times in the Book of Mormon. Eleven appear in conjunction with holy Nephite gatherings at their temples. Jacob tells of coming “up into the temple this day” to rid his garments of the people’s sins and to declare the word of God (Jacob 2:2-3). King Benjamin uses the phrase “this day” five times in his monumental speech, and each time it occurs at ritual and covenantal high points in the text.
In Hebrew the word etzem (“selfsame” from the phrase “this selfsame day”) is significant. Abraham Block has recently concluded that “this descriptive word was not a mere literary flourish” but a technical term of art with some unknown special significance. For further insight, Block turns to the medieval Jewish jurist Maimonides, who "noted with great amazement that etzem (selfsame) was used only in connection with the observance of Yom Kippur [the Israelite festival of the Day of Atonement] and Shavuot [the biblical festival of the Firstfruits, or Pentecost]. The implication is that this term was used to indicate that these high holy days in and of themselves produced a binding legal effect or holy religious status.
Evidently, in Nephite language and rhetoric, the phrase “this day” often indicated the covenantal and legal status of a holy day, much as “this day,” ”today,“ or ”this selfsame day“ did in Hebrew. [Donald W. Parry, John W. Welch, and Stephen D. Ricks, ”This Day," in Reexploring the Book of Mormon, pp. 117-119]