The three day lent of Nineveh commemorates the three days that Prophet Jonah spent inside the fish (for the story, see the Book of Jonah in the Old Testament). This fast is unique to the Syriac Orthodox Church which it passed to other Oriental Churches such as Coptic and Armenian. It is observed for three days from the tenth Monday before Easter Sunday.
Historically, this fast is one of the most rigorously observed fasts in the Syriac Orthodox Church. The faithful traditionally refrain from food and drink for three consecutive days, from Monday till Wednesday! Some observe the fast by refraining from food (and drink if they so wish) from morning till sunset during the three days. The church enjoins you to at least refrain from meat, fish and dairy products during the period of lent.
May our fasting and prayers accepted in the Highest.