The Eastern churches (both Catholic and Orthodox) traditionally observe a period of fasting prior to the Great Feast of the Dormition of the Theotokos - one of the four great fasts that are part of the Church year in the East, the others being the Nativity Fast, the Great Lent Fast and the Apostles Fast. This fast is also called "Our Lady's Fast" or the "Assumption Fast.” For those on the new calendar (Gregorian), this fast begins just before the Vespers of the Feast of the Procession of the Holy Cross (1 August) on 31 July evening. This fast ends just before Vespers for the Great Feast of the Dormition of The Theotokos (15 August) on 14 August evening. For those on the old calendar (Julian) , this fast begins on August 14 and ends on August 28.
The Dormition Fast is short, but is stricter than all the other fasting periods except Great Lent. One should fast on all days from the usual non-lenten foods, such as all animal products (meat, poultry, milk, cheese, etc.) and olive oil and wine. In addition, one also traditionally abstains from fish on all days of the fast, including weekends, except for the feast of the Holy Transfiguration of Our Lord, when fasting may be eased by having fish, wine and olive oil. On the two weekends which fall during the fast (Saturday and Sunday), the fast is also relaxed a little bit, and one may have wine and olive oil, but no fish. Please check with your priest or with your eparchial (diocesan) offices for specific information, of course. Actually fasting practice can and does vary between the particular Eastern churches.
The Scriptural foundation for the practice of Fasting is found in the Synoptic Gospels, when the Pharisees criticized the Apostles for not fasting, Jesus said to them, "Can the wedding guests mourn as long as the bridegroom is with them? The days will come, when the bridegroom is taken away from them, and then they will fast.” Our Lord, in this passage, was referring to his being taken to be crucified; but in the larger sense these words of the Lord are understood in terms of his Ascension into heaven, and his command to preach the Gospel, which can only be accomplished with prayer and fasting. The New Testament mentions the practice of fasting many times.
According to Eastern Christian teaching, by fasting we observe this time-honored Apostolic practice. We can also unite our lives more closely to the Mother of God through the Dormition Fast. By fasting, we can join in and show our appreciation for the sacrifices of the the Theotokos. The Theotokos sacrificed much to give birth to Holy God the Son within the Holy Trinity - Our Lord, God and Savior, Jesus Christ, to raise Him, and to be with Him during His ministry, His crucifixion, and His resurrection.
The Dormition of the Theotokos is a Great Feast of the Eastern Orthodox, Oriental Orthodox and Eastern Catholic Churches which commemorates the "falling asleep" or death of the Theotokos (Mary, the mother of Jesus; literally translated as God-bearer). It is celebrated on August 15 (August 28 for those following the Julian Calendar) as the Feast of the Dormition of the Mother of God.
In the Eastern churches, as in the language of Sacred Scripture, death is often called a "sleeping" or "falling asleep.” A prominent example of this is the name of this feast; another is the Dormition of Anna, Mary's mother. According to the ancient tradition of the East, the Orthodox and Eastern Catholics believe that Mary, having spent her life after Pentecost supporting and serving the nascent Church, was living in the house of the Apostle John when the Archangel Gabriel revealed to her that her repose would occur three days later. The Holy Apostles, scattered throughout the world, are said to have been miraculously transported to be at her side when she died. The sole exception was the Apostle Thomas, who was characteristically late. He is said to have arrived three days after her death, grief-striken, and asked to see her grave so that he could bid her goodbye. Mary had already been laid to rest. When they arrived at the grave, her body was gone, leaving a sweet fragrance. An angel is said to have appeared and confirmed to the Apostles that Christ had taken her body to heaven after her soul.As a result, the Dormition Fast is a period of preparation for the celebration of the Assumption of Our Blessed Lady into Heaven on August 15th. It is wise for us - even Latin Rite Catholics - to adopt this time as a time of prayer and preparation. While we are not canonically bound under pain of sin to observe this period of fasting, it is certainly a wise practice for us to willingly fast at this time in order to better prepare for the Assumption. And moreover, offering our fasting and added sacrifices for the intentions of our Lady would be a most salutary effort indeed.