Let us fast with a fast pleasing to the Lord. This is the true fast: the casting off of evil, the bridling of the tongue, the cutting off of anger, the cessation of lusts, evil talking, lies and cursing. The stopping of these is the fast true and acceptable.
—from Monday Vespers of the first week of Lent
Fasting in the Orthodox Church needs first of all to be differentiated from what is normally thought of as fasting. To fast for Orthodox Christians does not mean not eating at all (although this is also practiced—Orthodox Christians eat and drink nothing after Vespers on the day preceding a day on which the Divine Liturgy is celebrated until the eucharist itself is done), but means rather the abstention from certain foods—and often, from other distractions, such as television, parties, (non-liturgical) music, etc. It is important here to remember that, in contrast to some other non-Christian religions that emphasize fasting, no food is marked as forbidden in Orthodox thought—following the belief that all things are "made new" in Christ. The dream which St Peter relates of being told to eat even those animals deemed unclean in Jewish dietary laws is also cited. On the contrary, fasting is taken as a spiritual discipline and training for the spiritual athlete. It is a self-imposed restriction, or an obedience to one's spiritual father, of one's habits to remove from ourselves the tyranny of those habits—to allow us to rule our body rather than be ruled by it. Again, I want to stress that this doesn't mean that there's anything inherently bad or sinful about meat and dairy or about the body itself.
The basic rule, as it is practiced in monasteries, is that Wednesdays and Fridays are "strict fast" days, meaning that meat, dairy, fish, wine (which is extended to mean all alcoholic beverages), and (olive) oil are not allowed, although most monastics refrain from meat every day in any case, unless they are ill. In addition, the amount of food taken in is reduced to about one-and-a-half meals' worth. This is to purposefully reduce the body's energy level, with the idea of making it easier to pray by quieting and focusing the mind. The money saved from eating less also can be used to practice the virtue of compassion, by giving to the poor and needy.
There are also various fasting periods throughout the year, the most important, of course, being Lent. Lent—often called "Great Lent" in Orthodox practice, in opposition to "Little Lent" as the Nativity Fast is sometimes called—begins forty-seven days before Pascha—forty-seven because the week immediately preceding Pascha is counted as Holy Week, a thing unto itself. (The date of Pascha also, usually differs (except in Finland, where it's mandated by law) from the date of Easter celebrated by the Catholic Church and the various protestant churches because the Orthodox Church calculates Pascha (for reasons I won't get into because they're contentious) according to the Julian calendar. During Lent, the strict fast applies every day—and in keeping with the penitential character of the season, there is also a "eucharistic fast" throughout it, during which the Divine Liturgy is not served (remember, Saturday and Sunday don't count). The fast begins on a Monday, and the Sunday immediately before it is called Cheesefare Sunday, as it's the last day to eat dairy and such. However, abstention from meat begins the Monday of the week before, right after Meatfare Sunday.
The fast is not always so strict, however. Even in Lent, if there's a big feast day that happens to fall—for example, the feast of the Annuciation often falls during Lent—the fast is relaxed to allow fish, wine, and oil. And during Bright Week (the week following Pascha), the twelve days after Christmas, the week between the feast of Pentecost and the feast of All Saints, and the week prior to Meatfare Sunday, fasting is not prescribed at all, even on Wednesdays or Fridays. Some conditions preclude fasting: pregnant women and nursing mothers, small children, the elderly, the sick—after all, the goal is to help, not to harm. Some of the fasts themselves are simply relaxed ones, also.
Strict fasts are:
- Great Lent: beginning and ending vary; based on the date of Pascha
- Holy Week: week immediately before Pascha
- Dormition Fast: lasts from the first of August to the fourteenth. The Dormition is "the falling asleep of the Theotokos."
Relaxed feasts are:
- Nativity Fast ("Little Lent"): 15 November to 24 December
- Ss. Peter and Paul Fast: beginning is variable since it starts on All Saints, which is based on the date of Pascha, but it ends on 29 June
In addition, Mondays are sometimes kept as an additional fast day during the year—refraining from "fleshmeats" in honor of the Fleshless Ones; Monday is liturgically devoted to the remembrance of angels.
All that has been said thus far has been mainly for those under the monastic rule. For Orthodox Christians living in the world, the standard is the same, but can vary widely by parish practice. Generally, decisions on fasting are made by the person involved and his or her spiritual advisor, who is usually the parish priest, but not always. It is for them together to decide what would be helpful.