This is one of the 5 canonical lent in the Holy Church. This is observed three weeks prior to beginning of the Great Lent (Valia Nombu).
Bar Ebroyo narrates the history of this fast as follows in his Eccleciastical History:
“The people in the East began the fast, which is being observed three days after the Epiphany, in this time (i.e. the time of Catholicos Hananeshu, who became the Catholicos in A.D.686). When Abdul Malik the son of Morwon heard of the beauty of the women in Hirto, he asked the Governor there to send all the virgins to him. As this command came, the governor started to gather all the virgins. Then all the Christians came together in the church, and prayed and fasted. They prostrated before the Lord and appealed Him to save them from this punishment. On the third day when their Bishop John was reading the gospel he got a revelation about the death of Abdul Malik, and he revealed it to the people. After some days, this news was confirmed. It is said that, from this time onwards the believers observe this fast. But some others say that this fastwas begun in the East when the king Quzra Abroose prepared to rape the virgins.” Cf. Chronicon Ecclesiasticum III, cols. 139-141. (Abbeloos, J. B. and Lamy, T. J. (ed. and transl.): Gregorii Bar-Hebraei Chronicon Ecclesiasticum, I-III, Paris-Louvain, 1872-1877.)
Translating from the famous book “Bibliotheka Orientalis” by Assemani (ASSEMANI, J. S.: Bibliotheca Orientalis Clementino-Vaticana I, De scriptoribus Syris Orthodoxis, Rome, 1719 / reprint Hildesheim-New York 1975; II, De scriptoribus Syris monophysitis, Rome, 1721/ reprint Hildesheim-New York, 1975) Fr. Chediathgives the following versions also in his translation of Bar Ebroyo’s”Sabhacharithram” (pp.248-251)
A lot of people died in the village of Beth Garme because of a fast spreading infectious disease. The Bishop of that diocese Mor Sabarisho summoned the people for a public prayer. He said them ” The most virtuous God has showed mercy to the people of Nineveh and withdrew from the intended punishment according to the words of prophet Jonah. How much more will He show mercy to us, who believe in Him, His Messiah, and in His Holy Spirit?. And the people obeyed him. They came together and supplicated to God intensively. All, including the infants, have worn sack clothes. And God has removed the infectious disease for them. As theEast Syrian Catholicose Thimothi (he died in AD 819) took charge as catholicose, he ordered these 3 days prayer and fast to be continued annually. And this Lent should be known as the lent of Anger and not as the lent of Nineveh.
Let us observe the canonical lent, God will richly bless those who observe it.