Orthodox fasting is a change in kind of food rather than a reduction in quantity: on a strict fasting day meat, dairy, eggs, fish, olive oil and wine are all set aside, and the diet becomes plant based, which is why the practice is sometimes loosely described from the outside as a form of periodic veganism, though the tradition itself frames it as ascetic discipline rather than dietary ethics. Strictness varies through the calendar: fish, oil and wine are permitted on many otherwise fasting days according to the feast being kept, and the strictest days, such as the first week of Lent and Holy Week, allow far less.
Monastics generally keep a fuller version of the calendar than most laypeople are expected to, and married couples, the sick, travelers and small children have long been granted latitude by their priests, a pastoral flexibility that the tradition treats as built into the discipline rather than as a departure from it. What unites the practice across this variation is the belief that fasting is inseparable from prayer and almsgiving, a bodily discipline meant to serve repentance rather than an end in itself. The tradition has a name for the strictest form of this discipline, xerophagy, dry eating, food prepared without oil and taken only once a day on the most demanding days of the year, a standard monasteries generally keep closer to in full than most lay households are asked to. Historically the calendar took its settled shape across the Byzantine centuries, compiled into the Typikon alongside the rest of the liturgical order, and it remains one of the clearest points of continuity between the medieval Byzantine church and Orthodox practice today, observed with only modest local variation across the Greek, Slavic, Arab and other jurisdictions that share it.