Sunday of All Saints

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.

http://orthodoxwiki.org/Pentecostarion#Sunday_of_All_Saints