Why do the Gospel accounts emphasize that Jesus was dead three days? By my count, there are 26 references to the three days of Jesus' death in the New Testament. In Matt 12:40, Jesus gives us a hint about its significance: "For just as 'Jonah was three days and three nights in the belly of the whale,' so will the Son of Man be three days and three nights in the heart of the earth," quoting Jonah 1:17.
Another hint can be found in Luke 18:33, where Jesus says "they will kill him, and on the third day, he will rise." The phrase in italics is almost identical to Hosea 6:1-3 (in the LXX, the Greek Old Testament): "the Lord... has seized and he will heal us, he will strike and he will bandage us. He will heal us after two days, on the third day we will rise, and we will live before him."
These two Old Testament passages have something in common. Both of them, read in their orginal contexts, describe how God brought wrath on someone, but then rescued. Jonah cries out "I cried for help from the depth of Sheol [the grave]; you heard my voice... the earth with its bars was around me forever, but you have brought up my life from the pit" (Jonah 2:2-9). Hosea 6 in a similar way describes God's restoration of his people who experienced his wrath.
Jesus picks up this theme in his predictions of his death and resurrection. Like Jonah, Jesus would experience God's wrath, but after three days he would come up from "the depth of Sheol;" like Israel in Hosea, Jesus would be healed by the God who wounded. Jesus' prediction of "three days" was thus designed to recall these OT examples of faith that God would deliver; it probably also teaches that Jesus would suffer the wrath of God before his vindication and resurrection.
The picture above is a portrayal of the women at the empty tomb. It comes from The Brick Testament - a very interesting website, but occasionally rather warped.