According to Byzantine Catholic practice, on Sundays, Mondays, Tuesdays, Thursdays, and Saturdays one abstains from meat, meat products, and all dairy and dairy products, which includes eggs. Fish and shellfish are distinguished from each other in Eastern Christian dietary practice, are permitted on some days, as are olive oil and wine until 12 December. In something of a reversal on Fridays through 12 December, while fast days, there is no limit to the number of meals eaten on these days and both olive oil and wine are permitted. 12 December, which is the Feast of Our Lady of Guadalupe on the Roman calendar, marks the last day of the initial part of the Nativity Fast.
Beginning on 13 December, all days through 24 December are days of abstinence that one fasts from meat, meat products, all dairy, fish, olive oil, and wine. Shellfish is permitted. While it is important to take the fast seriously, in the United States, with our observance of Thanksgiving during the Nativity Fast, it is okay and I would say even encouraged to just go ahead and observe Thanksgiving in the normal and expected way. It is similar to how we Roman Catholics in the U.S. typically take a break from our strict Lenten observance on St. Patrick's Day. On Wednesdays and Fridays from 13-24 December it is customary to only eat one very modest meal. During the latter period of the Nativity Fast wine and olive oil are permitted only on Saturdays and Sundays.
As a Roman Catholic who for the most part observes Eastern Christian discipline (because we have largely abandoned such disciplines in the West) I make some modifications in keeping with my own tradition. Sundays are always feast days. So, there are no dietary restrictions on the Lord's Day. Also, I interpret no wine as no alcohol. Observing these lengthy periods of fasting is difficult, which is why the following tips from the website of Our Lady of Fatima Byzantine Catholic Church in San Francisco are necessary, the most salient of which I post:
1. The external observances of our Faith do not make us better than anyone else. No sense of superiority or exclusiveness should be allowed to enter into our practice.
2. Insofar as possible, it is best to fast quietly, without letting anyone know that you are fasting. This is clearly in line with Our Lord's teaching. When ordering at a restaurant, don't proclaim, "No meat for me, I'm fasting!" Just order the dish which accords with the fast.
3. Do not become discouraged if you are unable to keep the whole fast. The Evil Spirit is always on the lookout to fool us into giving up because we cannot do it all. Part of fasting is to learn our weakness and inability to save ourselves.
4. Remember that Fasting includes a) fasting from sin; b) additional spiritual reading and prayer; c) almsgiving and other works of Philanthropia ("the love of humankind"). Do not neglect these as you prepare for the Feast.
So, what to eat? I point you a great website with a lot of links to recipes that enable observance of both the Nativity Fast and fasting during the Great Lent: Sources for Orthodox Fasting. Also, this year, I am enthusiastic about another website, An Orthodox Kitchen.
"By feeling mild pangs of hunger, I realize that I am not sufficient to bring myself into being, or to sustain myself. The desire for food can be a sign to me of my even greater need for God. Every time I pass a 'Jack-in-the-Box' or other fast-food hamburger emporium and want to get an 'Ultimate Cheeseburger,' I am also reminded of the season of the year, of my commitment to the Lord, and that nothing is 'Ultimate' except God." It is an opportunity to make reparation for the times I have loved things more than God and to mortify my sensual desires, especially those I am prone to misuse and/or overuse, not seeking them for a greater good, but only to gratify personal desire. Things are not bad in and of themselves, but by misuse and overuse they can become detriments to me. There are always those who protest that fasting and other conscious deprivations "are artificial or external methods" of drawing close to God. Indeed, they can easily become such, but when done in the proper spirit of humility, in recognition of my need, these are effacaciuous means.