Nativity Fast different from Advent
In Christianity, the Nativity Fast—or Fast of the Prophets in Ethiopian Orthodox Tewahedo Church and Eritrean Orthodox Tewahedo Church—is a period of abstinence and penance practiced by the Eastern Orthodox Church, Oriental Orthodox Church and Catholic Church in preparation for the Nativity of Jesus on December 25. Ethiopian and Eritrean Orthodox Churches commence the season on November 24th and end the season on the day of Ethiopian Christmas which falls on 7th of January. The corresponding Western season of preparation for Christmas, which also has been called the Nativity Fast and St. Martin's Lent, has taken the name of Advent. The Eastern fast runs for 40 days instead of four (in the Roman Rite) or six weeks (Ambrosian Rite) and thematically focuses on proclamation and glorification of the Incarnation of God, whereas the Western Advent focuses on three comings (or advents) of Jesus Christ: his birth, reception of his grace by the faithful, and his Second Coming or Parousia.
The Byzantine fast is observed from November 15 to December 24, inclusively. These dates apply to the Eastern Catholic Churches, and Eastern Orthodox churches which use the Revised Julian calendar, which currently matches the Gregorian calendar. For those Eastern Orthodox churches which still follow the Julian calendar—the Greek Orthodox Patriarchate of Jerusalem, the Russian Orthodox Church, the Serbian Orthodox Church, the Georgian Orthodox Church, the Ukrainian Orthodox Church, the Macedonian Orthodox Church, Mount Athos, the Portuguese Orthodox Church, and all Old Calendarists, as well as some parishes of the Romanian Orthodox Church, of the Polish Orthodox Church, and of the Orthodox Church of America—the Winter Lent does not begin until November 28 (Gregorian) which coincides with November 15 on the Julian calendar. The Ancient Church of the East fasts dawn til dusk from December 1 until December 25 on the Gregorian calendar.
Sometimes the fast is called Philip's Fast (or the Philippian Fast), as it traditionally begins on the day following the Feast of St. Philip the Apostle (November 14). Some churches, such as the Melkite Greek Catholic Church, have abbreviated the fast to start on December 10, following the Feast of the Conception by Saint Anne of the Most Holy Theotokos.Fasting rules
In the Byzantine Rite, the fast traditionally entails fasting from red meat, poultry, meat products, eggs, dairy products, fish, oil, and wine. Fish, wine and oil are allowed on Saturdays and Sundays as well as most Mondays, Tuesdays and Thursdays, and oil and wine are generally allowed on Tuesdays and Thursdays, except in the Ethiopian Orthodox Tewahedo Church. 
The fasting rules also permit fish, wine and oil on certain feast days that occur during the course of the fast: Evangelist Matthew (November 16), Apostle Andrew (November 30), Great-martyr Barbara (December 4), St. Nicholas (December 6), St. Spiridon and St. Herman (December 12), St. Ignatius (December 20), etc.  The Nativity Fast is not as severe as Great Lent or the Dormition Fast.
As is always the case with Byzantine fasting rules, persons who are ill, the very young or elderly, and nursing mothers are exempt from fasting. Each individual is expected to confer with their confessor regarding any exemptions from the fasting rules, but should never place themselves in physical danger.
There has been some ambiguity about the restriction of fish, whether it means the allowance of invertebrate fish or all fish. Often, even on days when fish is not allowed, shellfish may be consumed.  More detailed guidelines vary by jurisdiction, but the rules strictly state that from the December 20 to December 24 (inclusively), no fish may be eaten.
The Eve of Nativity (December 24) is a strict fast day, called Paramony (lit. 'preparation'), on which no solid food should be eaten until Sirius is seen in the evening sky (or at the very least, until after the Vesperal Divine Liturgy that day). If Paramony falls on a Saturday or Sunday, the day is not observed as a strict fast, but a meal with wine and oil is allowed after the Divine Liturgy, which would be celebrated in the morning.