The Apostles’ Fast, also called the Fast of the Holy Apostles, the Fast of Peter and Paul, or sometimes St. Peter’s Fast,^[1] is a fast observed by Eastern Orthodox, Oriental Orthodox, and Eastern Catholic Christians. In the Byzantine tradition, the fast begins on the second Monday after Pentecost (the day after All Saints’ Sunday), whereas in the Coptic and old Syriac traditions, the fast begins on the first Monday after Pentecost. It continues until the Feast of Saints Peter and Paul on June 29.

Source Wikipedia.org