The Apostles’ Fast or "Summer Lent" is a summer fast that begins after All Saints Sunday (following Pentecost) and concludes on June 29, the Feast of Saints Peter and Paul. It commemorates the Apostles’ preparation for their missionary work after Pentecost and invites the faithful to share in that spirit of prayer, fasting, and witness. It is also related to what Jesus said in Matthew 9:15, in reply to the Pharisees’ complaint that the apostles were not keeping fast days according to the Law: But the days will come, when the Bridegroom shall be taken from them, and then shall they fast."
Dr. Alexander Roman says, "The Apostles’ Fast is one that was actually engaged in by the disciples of Christ following the Descent of the Holy Spirit at Pentecost. Having rejoiced throughout the fifty days following Pascha, the Resurrection of our Lord, God and Saviour, Jesus Christ, the Apostles began to prepare for their departure from Jerusalem to carry Christ’s message to the far corners of the world. As part of their preparation, they began a fast with prayer to ask God to strengthen their resolve and to be with them in their missionary undertakings.
"It is this fast that we ourselves participate in when we observe the Apostles’ Fast. But we do this not only because we honor the Apostles and their sacrifice. We do this by way of preparing ourselves to be sent as missionaries into our world to reflect and radiate Christ to those around us."
The summer fast once went from the day after All Saints Sunday to August 15, but the ByzantineChurch used the principle of “economy” (oikonomia) and broke it into the two separate fasts we have today.
The traditional Fast as described in the Typikon is one of strict abstinence on Monday, Wednesday, and Friday. In the United States, the Ruthenian Metropolia (to which the Eparchy of Phoenix belongs) recognizes this as a penitential season and observance is voluntary.