13th century icon of Ss. Peter and Paul (Belozersk
Having rejoiced for fifty days following Pascha (Easter), the Resurrection of Jesus Christ, the Apostles began to prepare for their departure from Jerusalem to spread Christ's message. According to Sacred Tradition, 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.
The scriptural foundation for the Fast is found in the Synoptic Gospels, when the Pharisees criticized the apostles for not fasting, Jesus said to them, "Can the children of the bridechamber mourn, as long as the Bridegroom is with them? But the days will come, when the Bridegroom shall be taken from them, and then shall they fast." In the immediate sense, Christ was referring to his being taken to be crucified; but in the wider sense it is understood in terms of his Ascension into heaven, and his commission to preach the Gospel, which can only be accomplished with prayer and fasting.
The tradition of the Fast has existed at least since Pope Leo I (461 AD), as is evidenced by his homilies, though it has subsequently been forgotten in the West. The Fast is thought to have been instituted out of thanksgiving to God for the witness of the apostles of Christ. With this Fast, believers express their thanks for the apostles' endurance of persecution during their mission.