The Tradition of fasting in anticipation of the Nativity of the Lord is not unique to the Eastern Churches - the Latin Rite of the Church had observed this practice for centuries. Latin Rite Catholics today may certainly still observe fasting during this time to spiritually prepare themselves for Christmas. In fact, the Roman Catholic Church also used to keep a 40 day fast leading up to Christmas!
Beginning with Vespers on November 15th, the Nativity Fast continues until just before Vespers on Christmas Eve. As with all periods of fasting, Fasting is forbidden on Sundays. Due to many popular feast days occurring between now and December 9th, many places began to modify the fast to begin on December 10th.
Latin Rite Catholics traditionally fasted on the Vigil of the Immaculate Conception (December 7th) and on the Vigil of the Nativity (December 24th). Because of the Tradition of Fasting on Christmas Eve in the Roman Catholic Church, that evening is traditionally observed by the Feast of 7 Fishes. Those two days could (and arguably should) still be observed by Roman Catholics. In years when these days fall on a Sunday, fasting is suppressed (or prior to the 1917 Code, it would have been moved up to the Saturday).
The fast's purpose is to spiritually prepare the soul for drawing closer to God. Along with our fasting, we must increase our own prayer life, almsgiving, and good works. Fasting without increased prayer should never be done.
The Guidelines from ByzantineCatholic.org for the Nativity fast in most Eastern Catholic Rites are as follows:
All days except Sundays, from November 15 to December 12:
• Abstinence from: All Meats, Dairy Products and Eggs – no animal products.
• No abstinence from: Shellfish, Grains, Vegetables & Vegetable Products, Olive Oil; Fruit, Wine On Sundays fish is allowed until the final week of the Nativity Fast. On Wednesdays and Fridays, the usual year-round restrictions apply.
December 13 to 24:
• Abstinence from: All Meat Products, Dairy Products, Eggs, Fish, Olive Oil, Wine
• No abstinence from: Vegetables & Vegetable Products, Fruits and Grains
On Wednesdays and Fridays, food should not be eaten between meals, and meals themselves should be moderate in size. It is often customary to eat only one meal a day. During the Nativity Fast, from December 13 to December 24 inclusive, the Fast becomes stricter, and olive oil and wine are permitted only on Saturdays and Sundays. Fish is not permitted from the 13th to the 24th.
The Guidelines for the Antiochian Orthodox Church in America:
The Nativity Fast is one of the four Canonical Fasting Seasons in the Church year. This is a joyous fast in anticipation of the Nativity of Christ. That is the reason it is less strict than other fasting periods. The fast is divided into two periods.
November 15th through December 19th: The traditional fasting discipline (no meat, dairy, fish, wine, and oil) is observed. There is a dispensation given for wine and oil on Tuesdays and Thursdays. Similarly, fish, wine, and oil are permitted on Saturdays and Sundays.
December 20th through the 24th: The traditional fasting discipline (no meat, dairy, fish, wine, and oil) is observed. There is dispensation given for wine and oil only on Saturday and Sunday during this period.
The Guidelines for the Orthodox Church in America further state:
“It should be noted that in the Fast of the Holy Apostles and of the Nativity of Christ, on Tuesday and Thursday we do not eat fish, but only oil or wine. On Monday, Wednesday and Friday, we eat neither oil nor wine…. On Saturday and Sunday we eat fish. If there occur on Tuesday or Thursday a Saint who has a [Great] Doxology, we eat fish; if on Monday, the same; but if on Wednesday or Friday, we allow only oil and wine…. If it be a Saint who has a Vigil on Wednesday or Friday, or the Saint whose temple it is, we allow oil and wine and fish…. But from the 20th of December until the 25th, even if it be Saturday or Sunday, we do not allow fish.”
Above all, this time of year, as we approach Advent, consider the End of Times and our own Judgment, and await the celebration of the Nativity of Christ, let us embrace some fasting. Fasting on Wednesdays and Fridays during this time is preferable to not fasting at all. Consult your spiritual director and consider undertaking more fasting, almsgiving, and prayer during this preparation time. And when Christmas comes, let us celebrate it joyfully and festively throughout January and until Candlemas on February 2nd. While the world celebrates too early and ceases celebrating on the 2nd day of Christmas, let us not make that same grave mistake.