Jonah is swallowed by a whale and lives three days and three nights in its stomach until he repents and does God's will.

Jonah in the Whale's Belly and Nineveh's Forgiveness 

The city of Nineveh lived in evil ways, far from the Lord. They were in this period in the 8th century a. C., in the reign of Jeroboam II. It was necessary to straighten their paths. God then chooses the Hebrew prophet Jonah , a man with knowledge of His laws, to bring his prophecy to those people: they should immediately cease their iniquities or he would destroy the whole city in 40 days (just as he did with Sodom and Gomorrah, destroyed in the face of his evil actions and absence of life change). Jonah's problem: he knew the wicked of Nineveh and wanted distance from them. He was afraid of taking divine words to those who hated the Divine. 

Jonah's Solution: Flee from God's Order, and the Ship. He went to the city of Joppa , paid for boarding to the city of Tarshish. Everything seemed to be settled, God would not see him (!). But, of course, He saw everything and in detail. Now, God's solution: he sent a strong storm that threatened to destroy the vessel. The sailors suspected that there was a culprit on board and "cast lots" to find out who it was, falling on Jonah. They threw him into the sea and the storm ceased. 

But God did not want his death, on the contrary. So he sent a whale to his prophet, who swallowed him whole. Jonas suddenly found himself inside the animal's stomach , causing him great fear. He stayed there for three days and three nights . He repented and began to pray to the Lord: 

3. "In my affliction, I called upon the Lord, and he heard me. From the midst of the abode of the dead, I cried out to you, and you heard my voice. (...). 5. "And I said: I was rejected from before your eyes. Will it still be given me to see your holy temple again?! (...) 10. "I, however, will offer a sacrifice with songs of praise, and I will fulfill the vow that I made. From the Lord comes salvation." (Jonah, 2) 

And God takes pity on Jonah, causing the whale to vomit him on dry land and orders him again to go to Nineveh and deliver His words to those people. After three days of walking he arrived and preached to everyone to change their lives because in forty days Nineveh would be destroyed if they kept their evil hearts. 

The Ninevites believed in the prophet Jonah. So from the least to the greatest of all in the city, that is, the king, they dressed in sacks (clothing of the poor) and fasted in the expectation of being forgiven. And God, in his infinite mercy, seeing his true intentions to change, forgave everyone in the city and did not destroy it. 

The lesson of the beautiful tree that grows one day and dies the next 

But Jonah, knowing the wickedness of the Ninevites, was indignant with the Lord for his immense goodness, even with their repentance, even asking Him to take his life. He left the city and set up a cabin in the east to see what would happen to the city. The Lord, then, to rescue his prophet's good humor, made a beautiful tree grow on the same day. tree where it was to give it a great shadow . It worked! Jonas rejoiced immensely with the showy and grandiose tree. But early the next day, God dries up the bush he had created, just as he casts strong rays of sunlight on Jonah, leaving him almost fainting. Angry and sorry for the dead tree, he asks God again to take his life. Then He answers you with a great lesson: 

10. “You had compassion on a bush,” replied the Lord, “for which you did nothing, which you did not make grow, which grew in one night and died in one night. of Nineveh, where there are more than one hundred and twenty thousand human beings , who cannot discern between their right hand and their left hand, and an innumerable multitude of animals?" (Jonah, 4) 

Jonah then understood the magnitude of his mistake and the reason for God's compassion and mercy! 

_{Reference: Bíblia Católica. Jonas, 1-4}