The Monday following Trinity is a feast day in honor of the Holy Spirit. This holiday was established by the Church "for the sake of the greatness of the Most Holy and Life-Giving Spirit, as one is (from) the Holy and Life-Giving Trinity", in opposition to the teachings of heretics who rejected the Divinity of the Holy Spirit and His consubstantiality with God the Father and the Son of God.
The Holy Spirit is the Third Person (Hypostasis) of the Holy Trinity, the true God, consubstantial and equivalent to the Father and the Son.
Like all Persons (Hypostases) of the Holy Trinity, the Holy Spirit has properties inherent only to God. Like all the Persons of the Holy Trinity, the Holy Spirit is equal in His Divine dignity to the Father and the Son. Like all Persons of the Holy Trinity, the Holy Spirit is consubstantial with Him, possesses a single Divine essence (nature) with the Father and the Son.