The Apostles' Fast varies in length from 2 to 6 weeks (15 to 49 days). It begins on the Monday following the Sunday of Pentecost and extends to the feast day of Saint Peter and Saint Paul on 5 Epip (July 12).
This fasting period commemorates the struggles of the holy apostles to preach Christianity to the world. Its length varies yearly depending on the date of Easter, which in turn determines the date of Pentecost.
The people who are fasting are allowed to eat vegan food and fish.

"When the Bridegroom shall be taken from them.....then they shall fast" (Matthew 9:15).
The Apostles' Fast is the oldest fast and the first one kept by the Christian Church. During the Apostles' Fast, the Holy Spirit spoke to them, "As they ministered to the Lord and FASTED the Holy Spirit said separate me Barnabas and Saul for the work whereunto I have called them.
And when they fasted and prayed and laid their hands on them, they sent them away" (Acts 13:2-3).