From Monday after the Sunday of All Saints, we start the Fast of the Holy Apostles.
The fast of the Holy Apostles is very ancient, dating back to the first centuries of Christianity. The oldest testimony regarding the Apostles Fast is given to us by St. Athanasius the Great (†373). In his letter to Emperor Constance, in speaking of the persecution by the Arians, he writes: "During the week following Pentecost, the people who observed the fast went out to the cemetery to pray." "The Lord so ordained it," says St. Ambrose (†397), "that as we have participated in his sufferings during the Forty Days, so we should also rejoice in his Resurrection during the season of Pentecost. We do not fast during the season of Pentecost, since our Lord Himself was present amongst 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). Christ’s presence was like nourishing food for the Christians.
This summer fast, which we now call the Apostles fast, was earlier called the fast of Pentecost. 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 fulfil the Law of God, which pronounces justification, not for works, but by grace.
After the extended feast of Pentecost, the fast is particularly needed in order to cleanse our mind by ascetic labours and to make us worthy of the gifts of the Holy Spirit.
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 through almsgiving and fasting.
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.