He is the source of sanctification and all life.

 The feast in honor of the Holy Spirit, “one of the Trinity of God, one in honor, one in essence and one in glory with the Father and the Son” is celebrated on the Monday after Pentecost. This feast has been established by the Church “for the majesty of the Most Holy and Life-creating Spirit, for He is one of the Holy and Life-originating Trinity”, in reaction to the teaching of the heretics who rejected the divinity of the Holy Spirit and that He is equal-in-honor with God the Father and the God the Son. “The Holy Spirit”, as it says in the Synaxarion, “is called the Comforter because He can encourage and calm us; we received Him “in place of Christ” and through Christ we have Him; He intercedes before God for us with unspeakable sighs as the Lover of mankind, interceding for us in exactly the same way as Christ, for He is the Comforter. Therefore the Holy Spirit is called the other Comforter; wherefore the Apostle says: we have the Comforter Jesus before God. He is called according to the unity in essence; wherefore in one sense “one and the other” we understand identical and consubstantial, and the words: “one and the other” is about various natures. This Holy Spirit is one with the Father and the Son in everything; He therefore accomplishes everything with Them, even the future resurrection, and everything that He wants, He creates, sanctifies, divides, renews, sends, makes wise, anoints the prophets, and briefly to say: does everything, being all-powerful, omnipotent, good, correct, and reigning. Through him is granted every wisdom, life, activity, as He is the source of sanctification and all life. In general He has everything that Father and the Son has “except for being unbegotten and begotten”, proceeding only from the Father). When the Holy Spirit was poured out on all flesh, then the world was filled with every kind of gift, and through Him all peoples were guided to the knowledge of God, and both any illness and any infirmity are driven out”).

In the services of this day the same hymns are sung as on the day of Pentecost, except that the Canon in glory of the Holy Spirit is sung at Compline. On the eve of the day of the Holy Spirit the All Night Vigil is not appointed (Ustav, Chap. 56 on the Temple). Matins is served with the Great Doxology, but without singing the Polyeleon; instead of “More Honorable” at Matins the Irmos of the 9th Ode of the Canon is sung). In the Liturgy the Deacon intones the “Introit”, as on the day of Pentecost; the Dismissal is the same as on the day of Pentecost.

 S. V. Bulgakov