In Helaman 14:20, Samuel the Lamanite prophesies concerning the Lord's death:
But behold, as I said unto you concerning another sign, a sign of his death, behold, in that day that he shall suffer death the sun shall be darkened and refuse to give his [note "his"] light unto you; and also the moon and the stars; and there shall be no light upon the face of this land, even from the time that he shall suffer death, for the space of three days, to the time that he shall rise again from the death.
The concept of "the space of three days" seems to correlate with what is recorded in the Bible. According to John Pratt, on many occasions the Savior prophesied that he would rise on "the third day" from his death (see Matthew 16:21; 17:23; 20:19; Mark 9:31; 10:34; Luke 9:22; 13:32; 18:33), while at other times the phrase "after three days" or "in three days" was used (see Mark 8:31; Matthew 27:63; John 2:19). It is clear that these phrases were equivalent in meaning because they were used interchangeably. (Compare Mark 8:31 with 9:31 and 10:34; also Matthew 27:63 and 64). On one occasion the Savior stated that he would be "three days and three nights in the heart of the earth." (Matthew 12:40)
It is clear from many passages in the Old Testament and rabbinic literature that the Jews counted inclusively -- that is, in counting three days after a Friday death, they would have counted Friday as the first day, Saturday as the second, and Sunday as the third. (See Luke 13:32-33; 24:7, 21, 45-48) Moreover according to Jewish custom, any part of a day was reckoned as an entire day (including the night); hence, part of Friday, all of Saturday, and part of Sunday would have been counted as three days and three nights. . . . Thus, it is proposed that the day of the Crucifixion was probably a Friday. [John P. Pratt, "The Restoration of Priesthood Keys on Easter 1836 -- Part 1: Dating the First Easter," The Ensign, June 1985, p. 61]