Fasting is one of the pillars of monastic life. We are taught its value by our Savior Himself who prepared for His confrontation with the Devil in the wilderness by a fast of forty days (Matt. 4:22).

St. John Climacus writes: “Fasting ends lust, roots out bad thoughts, frees one from evil dreams. Fasting makes for purity of prayer, an enlightened soul, a watchful mind, a deliverance from blindness. Fasting is the door of compunction, humble sighing, joyful contrition, an end to chatter, an occasion for silence, a custodian of obedience, a lightening of sleep, health of the body, an agent of dispassion, a remission of sins, the gate, indeed, the delight of Paradise.” (Ladder of Divine Ascent, step 14.)

Our Lord Who teaches us: “When you fast put oil on your head and wash your face, so that your fasting may not be seen by others but by your Father who is in secret; and your Father who sees in secret will reward you” (Matt. 16:17-18).

Our fasting must always be joined to prayer. This is why we are careful to maintain the link between the time of our fasts and the Church’ liturgical cycles. It is also for this reason that hospitality is more important than the strict observance of outward rules of fasting, as the fathers often teach us.

Fasting is an instrument to overcome the power of darkness present in the world now and it has a purification value; it has to be experienced to be understood and appreciated. It could be recommended twice a week if possible on Wednesdays and Fridays.

In the Old Testament and in the New Testament, we see many examples of fasting. Jesus fasted. According to tradition, fasting is encouraged especially in times of great temptation or severe trials. Certain devils, “can be cast out in no other way except by prayer and fasting,” said Jesus. (Mark 9:29)

“Let them, therefore, with fasting and with prayer make their adjurations, and not with the elegant and well-arranged and fitly-ordered words of learning, but as men who have received the gift of healing from God, confidently, to the glory of God. By your fastings and prayers and perpetual watching, together with your other good works, mortify the works of the flesh by the power of the Holy Spirit” – Two Epistles of Virginity, 12

“Before the baptism, moreover, the one who baptizes and the one being baptized must fast, and any others who can. And you must tell the one being baptized to fast for one or two days beforehand. Your fasts must not be identical with those of the hypocrites.” – Didache, 7

“This fasting … is very good, provided the commandments of the Lord be observed … First of all, be on your guard against every evil word, and every evil desire, and purify your heart from all the vanities of this world. If you guard against these things, your fasting will be perfect. And you will do also as follows. Having fulfilled what is written, in the day on which you fast you will taste nothing but bread and water; and having reckoned up the price of the dishes of that day which you intended to have eaten, you will give it to a widow, or an orphan, or to some person in want, and thus you will exhibit humility of mind, so that he who has received benefit from your humility may fill his own soul, and pray for you to the Lord.

If you observe fasting, as I have commanded you, your sacrifice will be acceptable to God, and this fasting will be written down; and the service thus performed is noble, and sacred, and acceptable to the Lord.” – Shepherd of Hermas, Book 3, Similitude 5, Chapter 3

“Let us fast, brethren and sisters, lest tomorrow perchance we die.” Openly let us vindicate our disciplines. Sure we are that “they who are in the flesh cannot please God;” not, of course, those who are in the substance of the flesh, but in the care, the affection, the work, the will, of it. Emaciation displeases not us; for it is not by weight that God bestows flesh, any more than He does “the Spirit by measure.” – On Fasting, 17

“Abba Isidore said, “If you fast regularly, do not be inflated with pride; if you think highly of yourself because of it, then you had better eat meat. It is better for a man to eat meat than to be inflated with pride and glorify himself” – Silence (EWTN link)

“This is the mark of Christianity: however much a man toils, and however many righteous deeds he performs, to feel that he has done nothing, and in fasting to say, “This is not fasting,” and in praying, “This is not prayer,” and in perseverance at prayer, “I have shown no perseverance; I am only just beginning to practice and to take pains”; and even if he is righteous before God, he should say, “I am not righteous, not I; I do not take pains, but only make a beginning every day.” – Abba Macarius the Great, Homily 26

“Fasting gives birth to prophets and strengthens the powerful; fasting makes lawgivers wise. Fasting is a good safeguard for the soul, a steadfast companion for the body, a weapon for the valiant, and a gymnasium for athletes. Fasting repels temptations, anoints unto piety; it is the comrade of watchfulness and the artificer of chastity. In war it fights bravely, in peace it teaches stillness.” – Homily on Fasting

“Fasting cleanses the soul, raises the mind, subjects one’s flesh to the spirit, renders the heart contrite and humble, scatters the clouds of concupiscence, quenches the fire of lust, and kindles the true light of chastity. Enter again into yourself.”- Sermon, On Prayer and Fasting, LXXII

Carthusian fasting

• Carthusian Statutes Chapter 2
• Jesus himself, God and Lord, whose virtue was above both the assistance of solitude and the hindrance of social contact, wished, nevertheless, to teach us by his example; so, before beginning to preach or work miracles, he was, as it were, proved by a period of fasting and temptation in the solitude of the desert;
  
• We cannot here pass over in silence a mystery that merits our deepest consideration; the fact that this same Lord and Savior of mankind deigned to live as the first exemplar of our Carthusian life, when he retired alone to the desert and gave himself to prayer and the interior life; treating his body hard with fasting, vigils and other penances; and conquering the devil and his temptations with spiritual arms.
  

• Carthusian Statutes Chapter 7
• Fasting and Abstinence
  Christ suffered for us, leaving us an example, that we should follow in his steps; this we do by accepting the hardships and anxieties of this life, by embracing poverty with the freedom of God’s sons, and by renouncing our own will. Moreover, in accordance with monastic tradition it is for us also to follow Christ in his fast in the desert, treating the body hard and making it obey us, so that the mind may flame with longing for God.
  The fathers keep abstinence once a week, normally on Friday. On that day they content themselves with bread and water. On certain days and at certain times of the year, they observe the fast of the Order, and take only one meal a day (cf. chap. 48).
  We should practice mortification of the flesh not merely out of obedience to the Statutes, but primarily to be freed from the tendencies of our lower nature and enabled to follow the Lord more readily and cheerfully. But if, in a particular case, or with the passage of time, someone finds that any of the aforesaid observances is beyond his strength, and that he is hindered rather than helped in the following of Christ, let him in a filial spirit arrange some suitable measure of relaxation with the Prior, at least for a time. But, ever mindful of Christ who calls, let him see what he can do; and what he is unable to give to God by common observance, let him offer in some other way, denying himself and taking up his cross daily.
  Novices, therefore, should be accustomed gradually to the fasts and abstinences of the Order, so that, under the guidance of the Novice-Master, they may prudently and safely tend towards the rigor of complete observance. He should teach them to be specially watchful not to make future fasting a pretext for over-indulgence at meals. So, let them learn to chasten by the spirit the misdeeds of the flesh, and to carry in the body the death of Jesus so that the life of Jesus may also be manifested in their bodies.
  In accordance with the practice introduced by our first Fathers and ever since observed with remarkable zeal, we exclude all eating and taking of meat from our way of life. This abstinence is to be observed as a distinguishing mark of the Order and as a sign of hermit austerity, in which, with God’s help, we intend to persevere.
  No one is to indulge in penitential practices over and above those prescribed by the Statutes without the knowledge and approval of the Prior. But, if the Prior wishes someone to have some additional food or sleep or anything else whatsoever, or, on the contrary, if he wishes to impose something difficult and burdensome, we have no right to refuse, lest, in resisting him, we are found to be in reality resisting not him but God, whose place he holds in our regard. For though many and diverse are the things that we observe, we cannot hope that any of them will profit us without the blessing of obedience.
  
• Carthusian fasting for the Fathers is usually on Fridays and consists of eating solely bread and water
  

Fasting and medical considerations

• The Complete Guide to Fasting: Heal Your Body Through Intermittent, Alternate-Day, and Extended
• Intermittent Fasting for Weight Loss w/ Jason Fung, MD