In the Eastern Orthodox Church, the Nativity Fast runs from November 15 until after Liturgy for Nativity and 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, and oil and wine are allowed on Tuesdays and Thursdays. The fasting rules permit fish, and/or 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 Orthodox 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, and 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. The Church strictly states that from the December 20 to December 24 (inclusively), no fish may be eaten.
In answer to the question on your mind… YES, in the Antiochian Orthodox Church in America there is a dispensation for Thanksgiving.