Four multi week fasts structure the Orthodox liturgical year. Great Lent, the longest and strictest, runs for forty days before Holy Week and Pascha and is preceded by preparatory weeks that gradually reduce what may be eaten. The Nativity Fast covers the forty days before Christmas, kept less strictly than Lent in most jurisdictions. The Apostles Fast runs from the Monday after All Saints Sunday to the feast of Peter and Paul, its length varying from year to year because it depends on the date of Pascha, and the two week Dormition Fast precedes the feast of the Dormition of the Theotokos in mid August.
On top of these come the weekly fasts: most Orthodox Christians abstain on Wednesdays, in memory of the betrayal of Christ, and on Fridays, in memory of the crucifixion, throughout the year outside a handful of fast free periods that follow major feasts. Taken together, a fully observant calendar spends roughly half the year, counting both the long fasts and the weekly days, under some degree of restriction, though pastoral practice everywhere allows for individual circumstances such as illness, pregnancy, age and travel. Great Lent itself is preceded by several preparatory weeks, among them the weeks named for the parable of the Publican and the Pharisee, the parable of the Prodigal Son, and Meatfare and Cheesefare Sunday, which step the diet down in stages rather than beginning the full fast without warning, meat first, then dairy, so that the body and the household are not asked to change everything at once. Even within Lent, fish is permitted on Palm Sunday and on the Annunciation, a reminder that the calendar is a deliberate structure rather than one blanket rule.