Catechism of the Catholic Church
a, b, c, and d. Para. 731: On the day of Pentecost when the seven weeks of Easter had come to an end, Christ's Passover is fulfilled in the outpouring of the Holy Spirit, manifested, given, and communicated as a divine person: of his fullness, Christ, the Lord, pours out the Spirit in abundance (Cf. Acts 2:33-36).
Para. 732: On that day, the Holy Trinity is fully revealed. Since that day, the Kingdom announced by Christ has been open to those who believe in him: in the humility of the flesh and in faith, they already share in the communion of the Holy Trinity. By his coming, which never ceases, the Holy Spirit causes the world to enter into the "last days," the time of the Church, the Kingdom already inherited though not yet consummated.
Para. 739: Because the Holy Spirit is the anointing of Christ, it is Christ who, as the head of the Body, pours out the Spirit among his members . . .