The fast of Nineveh is a three-day lent recognized by the Orthodox Church. During this fast, we remember the three days Prophet Jonah spent in the belly of a large fish as penance for his disobedience to God and the atonement of the sinful city of Nineveh.
During these three days, we should look at our lives and show remorse for our disobedient natures and follow in the footsteps of Prophet Jonah and the people of Nineveh. As with all other fasts/lents of the Holy church, the fast of Nineveh is observed by refraining from the intake of all dairy and meat products.
May the Lord God accept your prayers, repentance and fasting. May He reward you and shower His blessing upon you.