Three Day Fast of Nineveh
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 fasting.
The faithful are urged to go to church after this fast (or the following Sunday) and receive the Holy Qurbono (the Eucharist).
Hymns from the Shhimo (Syriac Orthodox Book of Simple Prayer)
The Ninevites trembled at the voice of Yonah, the son of Mattay, and took refuge in penitence by watching, fasting and prayer; and by tears and groans the sentence of judgment pronounced by Yonah concerning the destruction of Nineveh was annulled. Blessed be the Compassionate one who turned them from evil to good. (Thursday Evening Prayer)
God, who heard the prayer of the son of Mattay in the sea and commanded the mighty fish to cast him up in three days, hear our prayer and be reconciled with us and respond in your mercy to our requests; and if we have angered you, there are those who will reconcile you with us, the just who died for your love (Wednesday Morning Prayer).