The Pentecostarion is one of the most beautiful liturgical books in the Orthodox Church. It is used in worship for eight weeks a year, from the Resurrection of the Lord to All Saints Sunday. The hope of the Resurrection that it conveys, the joy of the presence of the Risen Christ amid Christians, and the festive light, springing from the light of the Resurrection, that permeates the hearts of the faithful during these days are central landmarks of the spiritual life for the entire liturgical year. However, the Pentecostarion didn’t exist in the Church from the beginning. Like all books of worship, it is the fruit of an evolution of almost a millennium, an evolution dependent on the gradual emergence of Christian hymnography. The present study traces in broad outline the emergence of this liturgical book, with references to the liturgical books to which it is indebted and to the moments of synthesis-reform that defined it. At the same time, starting from the specificity of Holy Saturday’s services and the theological-liturgical logic, we will try to show that the service that should open the book of the Pentecostarion is the Vespers united with the Holy Liturgy of Saint Basil the Great celebrated on Holy Saturday.