The Church calls us to keep this fast according to the example of the holy Apostles, who, having received the Holy Spirit on the day of Pentecost, prepared themselves to preach the Gospels to the whole world.
On the fiftieth day after His rising from the tomb, and the ninth day after His Ascension and sitting at the right hand of the Father, the Lord sent down the Holy Spirit upon all His disciples and Apostles on the day of Pentecost. This is one of the greatest feasts of the Lord. This is the completion of the new, eternal covenant with mankind. When the Holy Spirit came down upon the Apostles, the Spirit of Truth, the Spirit of wisdom and revelation inscribed the new law of Zion in place of the law of Sinai. The law of Sinai gave place to the grace of the Holy Spirit, which confirmed the law [of Moses], and bestowed strength to fulfill the Law of God, which pronounces justification not for works, but by grace.
We do not fast on Pentecost [or during the afterfeast, which is fast-free] because the Lord was with us during those days. He Himself said, Can ye make the children of the bridechamber fast, while the bridegroom is with them?(Lk. 5:34).
"After the extended feast of Pentecost, the fast is particularly needed in order to cleanse our mind by ascetic labors, and to make us worthy of the gifts of the Holy Spirit," writes St. Leo the Great. "After the present feast, which the Holy Spirit has sanctified by His descent, all the people usually keep a fast beneficially established for the healing of soul and body, and therefore requiring that we spend it with the appropriate reverence. For we do not doubt that after the Apostles' hearts were filled with the Spirit of truth promised to them from on high, amongst the other mystical heavenly teachings given by their Teacher, they were given also the teaching on spiritual continence, so that their hearts, purified by fasting, would be made capable of receiving gifts of grace… It is would not be possible to struggle in a pampered body and fattened flesh against the persecutions, and the fierce threats of the ungodly that lay ahead; for that which delights our outer man destroys our inner man, and the more a wise soul mortifies its flesh, the purer it becomes.
"Therefore, enlightening all the sons and daughters of the Church by their example and instruction, the teachers designated a holy fast at the onset of warfare for Christ, so that as we set out to battle against spiritual depravation, we would have temperance as our weapon, and thus mortify our sinful desires, for our unseen enemies and fleshless foes will not overcome us if we do not give ourselves over to fleshly lusts. Although the tempter constantly and invariably desires to do us harm, he remains powerless and ineffective when he does not find any side of us to attack… This is why the steadfast and saving custom has been established, after these holy and joyful days we have celebrated in honor of the Lord, Who rose from the dead and ascended into heaven, and after we have received the gift of the Holy Spirit, of keeping a fast.
"The custom of earnestly keeping the fast is necessary also in order to preserve those gifts imparted now to the Church from God. Having been made temples of the Holy Spirit, and having drunk more than ever the Divine waters, we must not submit to any desires, or serve any vices, so that the habitation of the virtues might not be defiled by any unclean thing. With God's help and cooperation, we can all achieve this, if only we will cleanse ourselves by fasting and almsgiving, strive to free ourselves from sinful impurity, and bring forth abundant fruits of love." Further, St. Leo, Pope of Rome writes, "In the Apostolic canons inspired by God Himself, the Church fathers have, at the inspiration of the Holy Spirit, established first and foremost that all virtuous labors begin with fasting.
"They have done this because God's commandments can be fulfilled well only when Christ's army is protected from all temptations of sin by holy abstinence. Thus, beloved, we must exercise ourselves in fasting especially at the present time, as we are commanded to do at the close of the fifty days that follow the Resurrection of Christ, up to the descent of the Holy Spirit, which we have solemnly observed.
"This fast has been given to us in order to preserve us from slackness, for it is very easy to become slack due to the long period in which we were allowed to eat various foods. If we do not cultivate the field of our flesh continually, thorns and thistles will easily grow there, and produce fruits suitable only for burning, and not for the harvest storehouse. Therefore, we are obligated now to scrupulously preserve those seeds that we received in our hearts from the Heavenly Sower, and take precautions so that the envious foe might not spoil what God has given us, and so that the thorns of vice would not grow in the paradise of virtues. We can only avoid such evil though almsgiving and fasting."
Blessed Simeon of Thessalonika writes that the fast was established in honor of the Apostles, because we have been vouchsafed many blessings through them, and they have been shown to us as doers and teachers of fasting, obedience… and temperance. Even the Latins witness to this, albeit against their will, honoring the Apostles by fasting in remembrance of them. But in accordance with the Apostolic canons composed by St. Clement, after the descent of the Holy Spirit, we celebrate. Then, beginning from the following week, we honor the Apostles who have instructed us to fast.
How Long is the Apostles Fast?
The duration of the Apostles fast depends upon the date of Pascha, and therefore it varies from year to year. It always begins with the end of the Triodion, or after the week of Pentecost [the Sunday of All Saints], and ends on June 28, if the feast of Saints Peter and Paul does not fall on a Wednesday or Friday.
The fast can be as long as six weeks, and as short as one week and a day.
Patriarch of Antioch Theodore Balsamon (seventh century) said, "All the faithful, monastics and laypeople included, are obligated to fast seven days or more before the feast of Saints Peter and Paul, and let those who do not fast be excommunicated."
What should we eat during the Apostles Fast?
The asceticism of the Apostles fast is less austere than the forty days fast of Great Lent. During the Apostles fast, the Church rubrics prescribe for three days of each week—Mondays, Wednesdays, and Fridays—abstinence from fish, wine, and oil, taking uncooked food at the ninth hour after Vespers. On the other weekday, abstinence from fish is prescribed.
Fish is allowed on Saturdays and Sundays, as well on the commemoration days of great saints, or on patronal feasts.
St. John of Kronstadt on fasting
Lord! How characteristic it is of the First Image [mankind —OC] to attract and assimilate images, to settle into them and reside in them—whilst for them, as being made in Thine image, it should be natural to strive with all love and zeal towards the First Image, and cleave to Him. But this greedy, pleasure-loving, flabby, and sluggish flesh of ours pulls us away from Thee. We need fasting and temperance, but we are passionate for pleasures. Strengthen us for abstinence!