The Orthodox Church has always placed great emphasis on fasting. We fast on Wednesdays because on this day the decision was made to arrest Jesus. We fast on Fridays because it is the day on which Jesus was crucified. Fasting helps us remember that these are special days in the history of salvation. Other periods of fast are during Lent, Advent (pre-Christmas fast), the first fifteen days of August, etc.
Orthodox Christians fast from meat and products derived from meat, i.e., milk, cheese, eggs, butter, etc.
The purpose of such fasting is threefold:
Jesus fasted. He tells us in the Bible, "When you fast ..." He does not say, "If you fast ..." He expects us to fast.
An early Christian, Aristides, wrote:
"If there is a poor person among the Christians and they do not have the means to help him, they fast two or three days and give the food they have saved through fasting to the hungry person."
Orthodox Christians are called upon to fast not only for reasons of self-control and prayer, but also for reasons of love: to deny ourselves something that we may share what we have saved with a needy person.
from Making God Real in the Orthodox
Light and Life Publishing, 1977, p. 51-52