On the day after every Great Feast, the Orthodox Church honors the one through whom the Feast is made possible. On the day following the Nativity of the Lord, for example, we celebrate the Synaxis of the Most Holy Theotokos (December 26). On the day after Theophany, we commemorate St John the Baptist (January 7), and so on.
Today we honor the all-Holy, good, and life-creating Spirit, Who descended upon the Apostles at Pentecost in the form of fiery tongues in fulfillment of the Lord’s promise to send the Comforter to His disciples (JN 14:16). That same Holy Spirit remains within the Church throughout the ages, guiding it “into all truth” (JN 16:13).
One of the hymns at Vespers on Saturday evening tells us that the Holy Spirit “provides all things. He gushes forth prophecy, He perfects the priesthood, ... He holds together the whole institution of the Church.”
At Vespers on the day of Pentecost, we hear that the Holy Spirit is “the Fountain of goodness, through Whom the Father is known, and the Son is glorified.” He is “the living Fountain of spiritual gifts” Who “purifies us from our sins.” It is by the Holy Spirit that “the prophets, divine Apostles, and martyrs are crowned.” He is the source of life and of sanctification.
In the services of this day, we sing the same hymns as on Pentecost, except the Canon of the Holy Spirit, which is sung at Compline. The Vigil is not prescribed for the eve of today’s feast. We sing the Great Doxology at Matins, but not the Polyeleos. The Irmos of the Ninth Ode (“Hail, O Queen, glory of mothers and virgins...”) is sung in place of the Song of the Theotokos (“My soul magnifies the Lord...”).
At the Liturgy, the priest or deacon chants the Entrance Verse (“Be exalted in Thy strength, O Lord. We will sing and praise Thy power.”) as on the day of Pentecost. “Holy God” replaces “As many as have been baptized....” The dismissal of Pentecost is also used.
This whole week is fast-free, and the Leave-taking of Pentecost occurs on Saturday.
Source: www.oca.org