JOEL - Jesus prophesied that the only sign he would give the Pharisees concerning his resurrection would be the sign of Jonah,

"for as Jonah was three days and three nights in the whale's belly; so shall the Son of man be three days and three nights in the heart of the earth." (Matt. 12:39-40).

These should not be understood as three full 24-hour days. The Jews began their counting of the daily hours with sunset; therefore the hour before sunset and the hour following belonged to different days.
The Jews wanted Christ's body to be buried before the Sabbath(which was Saturday) (John 19:31, Mark 15:42), suggesting that Jesus died and was interred during Friday afternoon. His body lay in the tomb, dead, during part of Friday (first day), throughout Saturday, or as we divide the days, from sunset Friday to sunset Saturday, (second day), and part of Sunday (third day) (See Jesus the Christ, James E. Talmadge, 1962, p. 697).
So according to the Jewish reconing of time Christ was in the earth for approximately three days; Friday, Saturday, and Sunday. The Book of Mormon records that a period of darkness coinciding with Christ's time in the tomb lasted for about three days. (3 Nephi 8: 20-23).
If you want to get real picky the only discrepency is that He was not in the earth for three nights. Because of this some have argued that He must have been crucified on Thursday or even Wednesday instead of Friday.
But if you think about it how could Jonah really know exactly how many days and nights went by while he was in the totally dark belly of the big fish (assuming that story is to be taken literally)? The use of the word "three days" might be more of a figurative expression of time rather than representing an exact period of hours and minutes. Figuratively, the number three represents that which is of God, comes from God, or is authorized by God. So the the term "three days" may just represent some ideal period of time designated by God. It seems to be used in this way over sixty times throughout the scriptures (eg. Ex. 8: 27, Num. 10: 33, Josh. 1: 11, 1 Chr. 21: 12, Matt. 15: 32, Luke 2:46, Acts 9: 9, 1 Ne. 2: 6, 1 Ne. 18: 13).