It was named as such out of the custom of calling something after the name of the one who instituted that thing. It is only God to Whom fasting is due. The practice of this fast means following in the footsteps of the Apostles (Heb. 13-7) who observed it in fulfillment of the Lord Jesus' words: "The days will come when the Bridegroom shall be taken from them and then shall they fast." (Matt 9:15). After the Ascension of the Lord Jesus to heaven and the outpouring of the Holy Spirit upon the disciples, they started to fast. In reference to this the following is stated in the Acts of the Apostles: "As they ministered to the Lord and fasted." (Acts 13:2)
The duration of this fast differed in length based on the Julian calendar of Easter. Starting accordingly on June 16 and ending with the commemorative of the two chief Apostles, St. Peter and St. Paul on June 29.