The Sunday following Pentecost is dedicated to All Saints, both those who are known to us, and those who are known only to God. There have been saints at all times, and they have come from every corner of the earth. They were Apostles, Martyrs, Prophets, Hierarchs, Monastics, and Righteous, yet all were perfected by the same Holy Spirit.
The Descent of the Holy Spirit makes it possible for us to rise above our fallen state and to attain sainthood, thereby fulfilling God’s directive to “be holy, for I am holy” (Lev. 11:44, 1 Peter 1:16, etc.). Therefore, it is fitting to commemorate All Saints on the first Sunday after Pentecost.
This feast may have originated at an early date, perhaps as a celebration of all martyrs, then it was broadened to include all men and women who had borne witness to Christ by their virtuous lives, even if they did not shed their blood for Him.