From: Matthew 12:38-42
The Sign of Jonah
 Then some of the scribes and Pharisees said to him (Jesus), "Teacher, we wish to see a sign from you."  But he answered them, "An evil and adulterous generation seeks for a sign; but no sign shall be given to it except the sign of the prophet Jonah.  For 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.  The men of Nineveh will arise at the judgment with this generation and condemn it; for they repented at the preaching of Jonah, and behold, something greater than Jonah is here.  The queen of the South will arise at the judgment with this generation and condemn it; for she came from the ends of the earth to hear the wisdom of Solomon, and behold, something greater than Solomon is here."
39-40. This sign the Jews were asking for would have been a miracle or some other prodigy; they wanted Jesus, incongruously, to confirm his preaching--given with such simplicity--by dramatic signs. Our Lord replies by announcing the mystery of his death and resurrection, using the parallel of the case of Jonah: "No sign shall be given to it except the sign of the prophet Jonah." Jesus' glorious resurrection is the "sign" "par excellence", the decisive proof of the divine character of his person, of his mission and of his teaching.
When St. Paul (1 Cor 14:3-4) confesses that Jesus Christ "was raised on the third day in accordance with the scriptures" (words which later found their way into the Nicene-Constantinopolitan Creed, the Creed used in the Mass), he must have had this passage particularly in mind. We can see another allusion to Jonah in the words our Lord spoke shortly before his ascension: "Thus it is written, that the Christ should suffer and on the third day rise from the dead" (Lk 24:45-46).
41-42. Nineveh was a city in Mesopotamia (modern Iraq) to which the prophet Jonah was sent. The Ninevites did penance (Jn 3:6-9) because they recognized the prophet and accepted his message; whereas Jerusalem does not wish to recognize Jesus, of whom Jonah was merely a figure. The queen of the South was the queen of Sheba in southwestern Arabia, who visited Solomon (1 Kings 10:1-10) and was in awe of the wisdom with which God had endowed the King of Israel. Jesus is also prefigured in Solomon, whom Jewish tradition saw as the epitome of the wise man. Jesus' reproach is accentuated by the example of pagan converts, and gives us a glimpse of the universal scope of Christianity, which will take root among the Gentiles.
There is a certain irony in what Jesus says about "something greater" than Jonah or Solomon having coming: really, he is infinitely greater, but Jesus prefers to tone down the difference between himself and any figure, no matter how important, in the Old Testament.
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.