“For as Jonah was three days and three nights in the belly of the large fish, so will the Son of man be three days and three nights in the heart of the earth.”
His first sign is typical of Scripture, it is something that will happen in the future (compare Exodus 3:12; Isaiah 7:14). The future will prove the present (Deuteronomy 18:21-22). It does, however, require faith. In it He describes two things that were incongruous. The first was that Jonah spent three days and three nights in the insides of a large fish (citing Jonah 1:17 to Jonah 2:1), figuratively in the very depths of the grave (Jonah 2:2; Jonah 2:6). Here was a sign indeed, a sign of what happened to the disobedient. But it was also a sign of how God could deliver, even from the grave, and it cannot be doubted that this sign as recounted to the Ninevites played a great part in their response, along possibly with the unearthly pallor that had resulted from his sojourn in the fish and contact with its juices. Jonah had been to them a very sign from God. The second incongruous thing was that ‘the Son of Man' would similarly spend three days and three nights in the body of the earth, prior to His coming to the throne of God. He too would be in the very depths of the grave. And when He arose He too would be altered (compare Matthew 17:2; Acts 7:55-56). So Jonah was a sign to his own generation and a foreshadowing of the greater Who was coming. But the second was incongruous because in Daniel 7:13-14, instead of going into the grave, the Son of Man was to come in the clouds of Heaven from earth to the throne of God. The Son of Man was not supposed to be buried. He was supposed to ascend in triumph. And therein lay the sign. What was deemed impossible would happen, and when it did happen let them take note. The One Who was to take the throne of Heaven would first of all be locked in the body of the earth for three days and three nights, before, like Jonah had, He came forth in triumph. The presumption behind this was that after three days and three nights He would somehow rise again, as Jonah had. Thus Jesus death, burial and resurrection would be the promised sign. And it would convince many. It even convinced Paul. See 1 Corinthians 15:3-8.
‘Three days and three nights.' To the Jews part of a day could be described as ‘a day and a night' equally to a full day because they did not reckon scientifically. They saw the part as encapsulated within the whole. For example, in c 100 AD a well known Rabbi stated, “a day and a night make an ‘onah (twenty four hour day), and the portion of an ‘onah is reckoned as an ‘onah”. Thus in Jewish terminology Friday to Sunday would be ‘three days and three nights'. Some, however, do consider that Jesus died on a Wednesday, seeing it as being on the day of preparation of the Passover sabbath rather than that of preparation of the weekly sabbath. This, however, would not tie in with the women seeking to anoint Jesus' body on the first day of the week, for had Jesus been crucified on the Wednesday they would have sought to anoint him when the festal Sabbath was over on the Friday. They would not have waited another two days until the body had putrefied.