In the year 420 B.C. the Jewish people resolved to please God by breaking off their ties to non-believing peoples. For a while, it worked well for them as they withheld themselves from pagan marriage customs. However, they went so far in avoiding strangers, that they forgot that foreigners were also God's children.
When they had gone too far in avoiding foreigners, God inspired a humorous Jewish story teller to make up the story of Jonah who hated all non-Jewish people.
In this story God ordered Jonah to go through the great city of Nineveh, calling out, "Forty more days and Nineveh will be destroyed!" Now, Jonah, who hated the people of Nineveh, was looking forward to that destruction. However, to his extreme disappointment, the people of Nineveh, from the king on down, did penance, and received God's pardon for their many sins.
Our Church gives this story today to let us know that however great our sins have been, if we are sorry, doing penance, God will forgive us.