This is a three days fast, Monday, Tuesday, and Wednesday. It falls at one time in January and at another in February. Thus it is one of the rotating fasts. This year fasted from Monday 10 – 12 February 2020 G.
The reason behind the decision of our Church Fathers, that this fast be observed is that as the people of Nineveh were saved from the wrath of God through prayer and fasting, so also the faithful will receive mercy and blessings through this fast.(Jona. 3:5-9; Mt.12:39).