In the collection of the homilies of St. Gregory Palamas, there is a footnote explaining a little about the history of celebrating All Saints day on the first Sunday after Pentecost.
“With the Sunday of All Saints, the first Sunday after Pentecost, the cycle of moveable feasts in the liturgical calendar of the Orthodox Church draws to a close. This cycle began with the Sunday of the Publican and the Pharisee . . . and had its epicenter the Sunday of Pascha (Easter), the Feast of Feasts. Originally dedicated to the Triumphant Martyrs . . . this feast was later expanded (probably during the reign of Leo VI the Wise, Emperor from 886-912) to include all the saints, both known and unknown, throughout the ages. As such, it is also the feast to which every Christian looks with earnest expectation, for it sets before us, in a concrete and graphic manner, the very purpose of our existence: to become ‘heirs of God, and joint-heirs with Christ’ (Rom. 8:17) . . .
Furthermore, the liturgical sequence of the feast of the Pentecostarion (a liturgical book containing the services from Easter Sunday to the Sunday of All Saints) is significant, as it points to the pattern of the manifestation of God the Holy Trinity in the divine economy, namely, that it is the Son and Word of God who in and through His saving work, culminating in the Sunday of Pascha, reveals God the Father to the world, that it is the Holy Spirit who bears witness to the full and perfect divinity of the Son (Sunday of Pentecost), and that it is the communion of the saints, that is the Church, who together affirms the consubstantiality of the Holy Spirit with the Father and the Son (Sunday of All Saints).” ( footnote in Saint Gregory Palamas, The Homilies, pp 580-581)
The fruit of Pascha and Pentecost are the Holy Ones of God, the Saints. The Resurrection of Christ opens Paradise to all the chosen of God, while the coming of the Holy Spirit enables us to live the holy life on earth. The existence of the saints is a witness to God’s plan of salvation for all humankind.