In the Gospel, Jesus is concerned about the current generation. He says they are evil and have no faith. They are looking for a sign, but Jesus himself is the sign and there should be no need for another.  He tells them there will be no sign but for the sign of Jonah. 

Now Jonah is, I think, a very weird story.  In a nutshell:  Jonah is a good enough fellow, knows his prayers, goes to church, but when God tells him specifically to go to Nineveh to warn the people there of their impending doom, Jonah refuses to go.  He runs away from God and from his duty and gets on a ship.  God brings up a storm and tells the shipmen to throw Jonah overboard. Being decent fellows they are loathe to do so, but Jonah is cast overboard and swallowed by the fish.  After three days he says, OK God, I’ll do whatever you want, just let me out of the fish.  And he is spit out on the shore of Nineveh so he can preach.  As soon as he tells the people to repent before the Lord, they do so immediately. They had been pagan.  They didn’t know any better, but as soon as they learn of the Lord they convert and repent right away to save their lives and their city.  So God accepts their repentance and spares the city.  This is the part that gets to me – so now Jonah is angry because God did not smite the city.  He says, you told me to tell them that the end is near, and then it wasn’t. I look like a fool.  I wish I was dead.

I think this is an odd sign.  Jonah is the epitome of the reluctant hero.  He tries to run from his duty; even after he does his duty successful,y he does not appreciate the outcome.  Even as Jonah was in the whale for three days so Jesus will be in the earth for three days.  And as the people of Nineveh repented at Jonah’s words, people will repent at the Judgment.  Jesus is greater than Jonah (of course, that bar is not too high) and greater than Solomon, and the conversion for Jesus will be greater than those.  If the people would convert even for Jonah, whose devotion was practically non-existent – who had to be physically forced into action, how much more would the people convert for Jesus who was beyond signs and beyond prophecy but was the fulfillment of all.