The opening prayer to the Holy Spirit said at the beginning of nearly every Orthodox service and personal prayer rule. From the feast of Pascha to Pentecost it is omitted, replaced by the Paschal greeting.
Orthodox · Patristic (1st–7th c.)
O Heavenly King
O heavenly King, Comforter, Spirit of Truth,
thou who art everywhere present and fillest all things,
treasury of good things and giver of life:
come and dwell in us,
cleanse us from every stain,
and save our souls, O good one.