Chapter 3:5 Then the people of Nineveh believed in God and they called a fast and put on sackcloth from the greatest to the least of them.
God had prepared the people of Nineveh and Jonah through obedience delivered the word. Notice that they called a fast in order to redirect their focus where it needed to be and also put on sackcloth which was a symbol of sorrow and repentance, it was a coarse, dark cloth unfit for normal wear.
Chapter 3:6 When the word reached the king of Nineveh, he arose from his throne, laid aside his robe from him, covered himself with sackcloth and sat on the ashes.
We do not know which king this was. It was either Adad-mirari III (810-783 B.C.) or Ashurdan (771-754 B.C.). Notice his repentance with the wearing of the sackcloth as well as sitting in ashes which was a symbol of helplessness and despair.