Whenever our calendar marks a day as “fast” (this year’s calendar has fast days in blue), this is what it means in terms of food:

1. We do not eat or drink the following:
1. Meat 
2. Fish (with backbone), fish without backbone and certain seafood is considered Lenten food
3. Dairy (milk, cheese, etc.) 
4. All other animal products (eggs, , with the exception of honey) 
5. Oil (traditionally this meant olive oil, but then there were no other oils in common use in the ancient Mediterranean world, so no frying then!) 
6. Alcohol 
7. We don’t eat or drink anything which has even the smallest amount of the above!
2. We also eat less. Of course, less of what is allowed, mentioned above. For example, we should cut a three-meal per day diet to two meals and also reduce the quantity of food eaten per meal.
3. Traditionally, all the money saved from this change is given to the poor. 

Special note should be made of the fact that today, especially with the current vegan and vegetarian trends, one can “fast” on really good and expensive food, even “fast” sumptuously. So it is worth mentioning that eating well defeats the purpose of fasting and should be avoided. 

There is also a special fast, called “complete” or “strict” (also marked on our calendars as "strict fast"). A strict fast means that one abstains from all food and all drink (including water) for a certain time. It is this fast that we observe from midnight (at least) before communing at the Divine Liturgy in the morning, until after we receive Communion. Also certain special days over the year are marked as strict fasts. On these we don’t drink or eat anything for 24 hours. 

When the calendar marks a fast day with words such as “oil” or “wine,” this means that we are given a blessing to partake of those things. By the way, “wine” means alcohol in general, although hard liquor should be avoided. 

I brought up this physical aspect of fasting first, the non-eating, because it is the most visible. But it is not the most important. Also, thinking of fasting as not eating is not truly accurate, for a reason I will explain shortly. But first, let me point out that there are other physical aspects of fasting. As St. Basil says, we should “fast not only with our mouth, but with our eyes, our ears, our feet, our hands, and all our members.” St. John Chrysostom famously said that it is of no use to give up the meat of animals while biting—in malice and anger—into the flesh of our fellow human beings.

Yet, even more importantly, to fast means to move away from a physical manner of being to a spiritual one. The physical aspects of fasting do not serve a physical purpose, but rather the spiritual purpose of moving us past the material life. It is essential to understand this so that fasting is not a burden to us, but a joy. Indeed, to fast does not mean not to do something, but rather to do. In fasting we don’t truly deprive ourselves of things, but we fill ourselves up with things. When we fast we actually feast; we fast from the best material food so that we feast on what otherwise we may not be fully aware of. Just like the Lord said, one lives not only on bread, but also on God. When we fast we shift our hunger (our digestive attention, if you want) away from material things to immaterial. When we fast we eat the fruits of the world to come, we partake of the banquet in our Father’s house. We put one foot into paradise, eating the food of the angels. This is the reason for which fasting also means a heightening of our spiritual struggles, of patience, silence, acceptance of others as they are, etc. This is the reason for which during Lent we repeat very often the prayer of St. Ephrem: 

Lord and Master of my life, do not give me a spirit of sloth, meddling, love of power, and idle talk. But rather grant me, your servant, a spirit of sobriety, humility, patience, and love. Yes, Lord and King, grant me to see my own faults and not to condemn my brother; for you are blessed to the ages of ages. Amin.

Finally, let me address one question which would occur to us naturally: Why is it important to fast? Let me put it bluntly: the Church fasts. And the Church stretches not only between heaven and earth, thus including all the holy ones of our times, people and angels alike, but also she goes up and down in time, so she includes the ones who have come before us in the faith and the ones who will come after us. All these people have fasted, are fasting, and will fast. God willing, our children and our children’s children will fast. If we don’t fast, our communion with them and with Christ weakens, our being members of the body of the Church falters and breaks. And we separate ourselves from them at our own peril, since our strength is in our communion with each other. This is the terrible thing: not what we put in our mouth per se, but rather what that which we put in our mouth does to us. We cut ourselves off from our home, from our ancestors and from our descendants. And we end up in the embrace of the ego, that great wall between us and God, and between us and our fellow human being. So it could be said that we fast in order to discover that we are someone else's servants and we are not masters of ourselves. This realization is the health of our hearts and minds. 

This is how our tradition sees things and this is why we cannot accept other practices of giving up things by one’s own choice. That was another question I had: can we choose what we give up? Indeed other traditions do that, but that is not our tradition.