"It was Pentecost, fifty days after Easter, according to St. Luke in the Acts of the Apostles. That day was a momentous one, for, “On that day, [in the Upper Room] the Holy Trinity is fully revealed” (CCC 732). The Holy Spirit had been revealed gradually, and was the last of the Persons of the Trinity to be revealed, although the Spirit has from the beginning been a part of God’s plan of salvation for us.

That day, though, the Spirit did not come quietly! We read: “And suddenly there came from the sky a noise like a strong driving wind, and it filled the entire house in which they were. Then there appeared to them tongues as of fire, which parted and came to rest on each one of them. And they were all filled with the holy Spirit and began to speak in different tongues, as the Spirit enabled them to proclaim” (Acts 2:2–4). This 'fullness' of the Spirit was unlike anything they had ever experienced before."

—from Meeting God in the Upper Room: Three Moments to Change Your Life