Since ancient times, the 50-day period from Pascha to Pentecost has been called Pentecost because what began with the Lord breathing the Holy Spirit on His Disciples was consummated with the full descent of the Spirit upon the Disciples and the whole Church. Thus, the Church was fully born and began to grow.
Later on, another week was added to these 50 days in order to celebrate the post-feast of the Feast of Pentecost. Thus, today the period of movable Feasts after Pascha spans eight weeks, to include the Sunday of All Saints. READ MORE HERE