On Wednesdays and Fridays Orthodox Christians have "fasting days". Also, at various times during the Church calendar year, specifically the 40 days before the Nativity, the forty days (not counting but also including weekends and Holy Week) before Pascha (Easter), the Dormitian Fast in August (first two weeks of August) and the Apostle's Fast before the feast of Sts. Peter and Paul, which varies in length from year to year depending on when Pascha falls.
In total we are fasting about 51% of the year, I think.
Rather than figure all of this out, we usually buy handy calendars (available in places such as Parish bookstores, etc.) that have the fasting days clearly marked.
How we fast:
The typical Orthodox fast is a fast from meat products, eggs, dairy products, olive oil and wine.
Not everyone keeps the full fast. Some only abstain from meat. Some might abstain from meat and cheese but allow an egg or some tuna fish if they have extra protein needs, etc. What we all MUST fast from is judging others, and from sin, as much as possible. Therefore we never look at what is on someone else's plate. Lord have mercy.
Since Orthodox Christianity originated in the Mediterranean area, certain foods that were considered "the dregs" are allowed. Namely, spineless seafood (clams, oysters), crustaceans and insects (lobster, oysters, locusts...things with exoskeletons...eeeeew) and shrimp.
Now in America it is intersting because shrimp, lobster, crab, for instance, are considered a delicacy and cost more than chicken. So we walk a fine line of "this is canonically legal but ridiculous to spend more money on" and perhaps we eat tuna and fishsticks instead and save the shrimp for if it's lent and we are having company. The Russian practice is to allow fish during fasting periods, so we have the further complication of "Which culture to we look to as we work out our American Orthodox practice?"
So a fully fasting lenten diet will be vegan with shrimp and oysters and clams, crabs, lobster. I know that monastics typically always eat this way, with fish added for times of feasting, I think.
So we get good at making lentils and hummus, and things like this. And there's plenty of grace. People with health problems that are affected by diet need to follow doctor's orders first of all. A person's priest will advise on the individual fasting rule and keep one accountable.
There is also the idea that you don't eat extra and snack extra and only eat just enough to keep you going, during times of fasting. Lord have mercy.
But the point is, we aren't making up stuff willy nilly and we are fasting as a community. The first century writing "The Didache" refers to fasting on Wednesdays and Fridays as being the Christian practice, so we know that it dates back to the very earliest days of the Orthodox Christian Church. On Wednesdays we remember Christ's betrayal, and on Friday's we remember his crucifixion, just as on Sunday's we commemorate the resurrection of Christ.
There are certain days of the year, such as Great and Holy Friday, when a total fast is called for. Also, we fast from food and drink completely starting at midnight before the Eucharist on Sunday morning.
If a feast falls in the middle of a fasting period, such as the Annunciation on March 25, which is always during lent, fish, wine and olive oil are allowed. Additionally there are many "fish days" during the fasting period leading up to Christmas.
That's just the food fast. I won't even touch on other areas. But during lent many do without movies, computer, going out, music, blogging, reading materials get more sober and spiritual, etc. It all needs to challenge us to grow, to stretch ourselves, and to knock out some of our passions. It's a struggle and it is very difficult.
Now I know I've left out salient factors in this little "quick and dirty" run down of Orthodox fasting practices. I was told once that it can take seven years to learn how to fast. I know it's difficult. And mostly we fast so that we can pray with more focus, pray more, free up funds for almsgiving, and so that our passions can be curbed.
Here is the Prayer of St. Ephraim which we add to our prayer rule during Great Lent:
O Lord and Master of my life
do not give me the spirit of sloth, despair, lust of power or idle talk. prostration
But give rather a spirit of chastitiy, humility, patience and love to your servant. prostration
Yes, Lord and King, grant me to see my own transgressions
and not to judge my brother.
For blessed art Thou unto ages of ages. Amen. prostration