Adapted from an Wikipedia article
The number of fast days varies from year to year, but in general the Orthodox Christian can expect to spend a little over half the year fasting at some level of strictness. There are spiritual, symbolic, and even practical reasons for fasting. In the Fall from Paradise mankind became possessed by a carnal nature; that is to say, he became inclined towards the passions. Through fasting, Orthodox Christians attempt to return to the relationship of love and obedience to God enjoyed by Adam and Eve in Paradise in their own lives, by refraining from carnal practices, by bridling the tongue (James 3:5-6), confession of sins, prayer and almsgiving.
Fasting is seen as an exercise in self-denial and Christian obedience that serves to rid the believer of his or her passions (what most modern people would call “addictions”). These are often low-intensity and hard-to-detect addictions to food, television or other distracting entertainments, sex, or any kind of self-absorbed pleasure-seeking are seen as some of the most significant obstacles for man seeking union with God (Theosis). Through spiritual struggle (ascesis) the believer comes face to face with the reality of his weak and sinful condition — the starting point for genuine repentance.
All Orthodox Christians are expected to fast following a prescribed set of guidelines. They do not view fasting as a hardship, but rather as a privilege and joy. The teaching of the Church fixes both the times and the amount of fasting that is expected as a minimum for every member. For greater ascesis, some may choose to go without food entirely for a short period of time. A complete three-day fast at the beginning and end of a fasting period is not unusual, and some fast for even longer periods, though this is usually practiced only in monasteries. However, there are circumstances where a dispensation is allowed out of physical necessity: those who are pregnant or infirm, the very young and the elderly, or those who have no control over their diet, such as prisoners or soldiers. Fasting does not involve hatred of the body, but rather a desire to discipline and sanctify the body (Romans 6:12, Romans 8:5-8, I Corinthians 6:12-20, I Corinthians 9:27, etc.) and tame the flesh (i.e., that part of the personality which is addicted to the passions).
In general, fasting means abstaining from meat and meat products, dairy (eggs and cheese) and dairy products, fish, olive oil, and wine. Wine and oil—and, less frequently, fish—are allowed on certain feast days when they happen to fall on a day of fasting, but animal products and dairy are always forbidden on fast days. Married couples also abstain from sexual relations on fast days, that they may devote themselves to prayer (I Corinthians 7:5).
The time and type of fast is generally uniform for all Orthodox Christians; the times of fasting are part of the ecclesiastical calendar, and the method of fasting is set by the Holy Canons and Sacred Tradition. There are four major fasting periods during the year. They are:
The Nativity Fast (Advent or “Winter Lent”) which is the 40 days preceding the Nativity of Christ (Christmas), beginning on November 15 and running through December 24. This fast becomes more severe after December 20, and Christmas Eve is observed a strict fast day.
Great Lent which consists of the 6 weeks (40 Days) preceding Palm Sunday, and Great Week (Holy Week) which preceding Pascha (Easter).
The Apostles’ Fast which varies in length from 8 days to 6 weeks. It begins on the Monday following All Saints Sunday (the first Sunday after Pentecost) and extends to the Feast of Saints Peter and Paul on June 29. Since the date of Pentecost depends on that of Pascha, and Pascha is determined on the Julian Calendar, this fast can disappear completely under New Calendar observance (This is one of the objections raised by opponents to the New Calendar).
The Dormition Fast, a two-week long Fast preceding the Dormition of the Theotokos (repose of The Virgin Mary), lasting from August 1 through August 14.
In addition to these fasting seasons, Orthodox Christians fast on every Wednesday (in commemoration of Christ’s betrayal by Judas Iscariot) and Friday (in commemoration of his Crucifixion) throughout the year. Monastics often fast on Mondays (in imitation of the Angels, who are commemorated on that day in the weekly cycle, since monastics are striving to lead an angelic life on earth, and angels neither eat nor drink).
Orthodox Christians who are preparing to receive the Eucharist do not eat or drink at all from midnight until after taking Holy Communion. A similar total fast is expected to be kept on the Eve of Nativity, the Eve of Theophany (Epiphany), Great Friday and Holy Saturday for those who can do so. There are other individual days observed as fasts (though not as days of total fasting) no matter what day of the week they fall on, such as the Beheading of St. John the Baptist on August 29 and the Exaltation of the Holy Cross on September 14.
Strict fasting is canonically forbidden on Saturdays and Sundays due to the festal character of the Sabbath and the Resurrection, respectively. On those days wine and oil are permitted even if abstention from them would be otherwise called for. Holy Saturday is the only Saturday of the year where a strict fast is kept.
There are also four periods in the liturgical year during which no fasting is permitted, even on Wednesday and Friday. These fast-free periods are:
The week following Pascha (Easter)
The week following Pentecost
The period from the Nativity of Christ up to (but not including) the Eve of Theophany (Epiphany). The day of Theophany itself is always fast-free, even if it falls on a Wednesday or Friday.
The week following the Sunday of the Publican and the Pharisee (one of the preparatory Sundays before Great Lent). This is fast-free to remind the faithful not to boast like the Pharisee that he fasts for two days out of the week Luke 18:12).
When certain feast days fall on fast days, the fasting laws are lessened to a certain extent, to allow some consolation in the trapeza (refectory) for the longer services, and to provide an element of sober celebration to accompany the spiritual joy of the feast.
It is considered a greater sin to advertise one’s fasting than to not participate in the fast. Fasting is a purely personal communication between the Orthodox Christian and God. If one has health concerns, or responsibilities that cannot be fulfilled because of fasting, then it is perfectly permissible not to fast. An individual’s observance of the fasting laws is not to be judged by the community (Romans 14:1-4, but is a private matter between him and his Spiritual Father or Confessor.