Typikon on the Nativity Fast

For these 40 days we must keep three days of every week for fasting from oil and wine - on Monday, Wednesday and Friday, unless there is a major saint, in whose memory we allow it for the sake of our love for him and because of his feast, which falls in this month on: 16, 25, 30, and in December on the 4, 5, 6, 9, 17, 20. In those days like on Tuesday and Thursday we eat fish. On Monday, Wednesday and Friday we allow only oil and wine but we do not eat fish, unless it is the parish feast; if there is a church in the monastery dedicated to one such day\saint, we allow fish and wine. Whatever the day the Entry of Theotokos falls on, including Wednesday and Friday, we allow fish.

Same thing more generally:

It should be known that during the Fast of the Holy Apostles and of the Nativity, on Tuesdays and Thursdays, we do not eat fish but only oil and wine. On Mondays, Wednesdays and Fridays we do not consume oil, nor wine but we fast until the 9th hour and eat raw food on those days. On Saturdays and Sundays we eat fish. If there a saint day that has doxology falls on Tuesday or Thursday, we eat fish; if this occurs on a Monday - same thing; if it falls on a Wednesday or a Friday, only oil and wine is allowed and eat once during the day. If the saint has a vigil, we allow oil, wine and fish on Wednesdays and Fridays. If there is a commemoration of a saint, whose church it is, we do the same  on Wednesday and Friday.

Nativity Fast according to the Typikon

Translation of a Russian article about fasting according to the Typikon (auth. E. Kovina)

According to the Typikon, it is conducted the same way as the Fast of Sts. Peter and Paul. When the so-called "Alleluia service" (i.e. service that is strict and is akin to services during the Great Lent)* is conducted and there is no liturgy, one is to eat raw food at the 9th hour. The first day both the Nativity and the Petrine Fast, unless it falls on a Saturday or a Sunday, such service has to be performed. On other days of these fasts when commemorating minor saints, the choice is left to the parish priest. The Typikon specifies feasts, on which there is a polyeleos or a vigil and one can eat twice and have wine and oil, on the following days: 16, 25 and 30 November and 4,5,6,9,17, 20 December (Julian Calendar). Days that commemorate Russian saints are also added.* On the Eve of the Nativity, i.e. 21st of December (JC) fish is no longer allowed even on Saturdays and Sundays. During the eves of the Nativity and Epiphany one is to fast from food and drink until the evening. One is to eat cooked food with oil once a day after the Vespers, i.e. no earlier than the 9th hour (3pm). If those days fall on a Saturday or a Sunday, in order avoid fasting (i.e. full abstinence from food\drink), after the Liturgy of Saint John Chrysostom at the 6th hour (12pm), one can eat "a piece of bread and a little wine". After the Vespers "we eat normally without fish but with oil and boiled porridge or Kutia with honey; we drink also wine or in poorer countries - beer".

*See your Horologion for details or check here: http://www.orthodox.net/ustav/alleluia-days.html It should be noted, however, that at least in MP this tradition is almost fully abandoned when it comes to lesser lenten periods and only a handful of parishes still keep it.

**If one is Greek - Greek saints, Georgian - Georgian saints, etc.

Regarding the Nativity Fast

For those interested in going old-school, here's an excerpt from the so-called Small Home Rule, which is a compilation from various books for lay people on things related to praying and fasting.

From the Great Rule. From the day of holy apostle Phillip until the day of our holy father Nicholas one is to eat raw food on Monday, Wednesday and Friday. On Tuesdays and Thursdays as well as on Saturdays and Sundays one can eat fish. From the day of our holy father Nicholas until Christmas, we eat fish on Saturdays and Sundays, while on Tuesdays and Thursdays we consume cooked food with oil and we also drink wine.

From the Nomocanon. Before Christmas fish is allowed during  the fasting period only on Saturday and Sunday, while on Tuesday and Thursday oil and wine is allowed. On Monday, Wednesday and Friday we fast until the 9th hour (i.e. 3pm). If someone cannot drink water on Monday, oil and wine is allowed. Wednesdays and Fridays should be observed as strict as one's strength allows, save for great and burdonsome ailment.

From the Rule of Solovetsky Monastery. During the Christmas fast until the day of Saint Nicholas the Wonderworker, one is to eat fish on Saturdays and Sundays, on Tuesdays and Thursdays. On Saturday and Sunday - twice*, on Tuesday and Thursday - once during the day. On Monday shchi (simple cabbage soup) is eaten with one cold dish and one hot one. On Wednesday and Friday shchi is to be eaten along with two cold dishes. From the day of Saint Nicholas until the Christmas Eve we eat Plasti (dish with cold fish), on Monday, Wednesday and Friday we add two cold dishes to Shchi; on Tuesday and Saturday we add one cold dish and one hot dish to Shchi. On Christmas Eve on Saturday and Sunday we eat with oil twice a day; on other days we eat as during the Great Lent.

*Fasting also includes the number of dishes eaten. Unless specified otherwise - it is assumed that one eats twice a day, either after the 6th (12pm) or the 9th hour and then after the Vespers (the choice will depend on the way services are conducted). Breakfast is considered a sort of vanity and ideally people are to pray in the morning instead of eating.