Mt. 12:40-41

"For as Jonah was three days and three nights in the belly of the great fish, so shall the Son of man be three days and three nights in the heart of the earth. The men of Nineveh shall rise in judgment with this generation, and shall condemn it; because they repented at the preaching of Jonah; and, behold, a greater than Jonah is here."

I'm reading through Matthew (in case you were wondering), and I must confess I've never read it before with a real understanding of the term "kingdom of heaven," a phrase found only in Matthew. I understand it now (I think) to mean the Church Age, and it's a whole new world. See 12:16-21: Jesus would not be taken prematurely by His enemies, and He would not be made King by His followers (Jn. 6:15), because His mission was to extend grace to the gentiles. The kingdom of heaven, announced as at hand by the Baptist and by Christ, is the period of time when God demonstrates He is not just God of Israel but sovereign Lord over all His creation. He does this by creating a new nation, not just a physical people on the eastern shore of the Mediterranean, but a vast people, Jew and gentile, across the entire world - His Church, the Israel of God (Gal. 6:16). Keeping this in mind while reading Matthew is a revelation.

So what of the sign of Jonah? The prophet spent three days and three nights in the belly of the fish for one reason: so gentiles would be saved.