Pentecost

"When the Paraclete comes, whom I shall send to you from the Father, the Spirit of truth who issues from the Father, he will be my witness. And you too will be my witnesses, because you have been with me from the beginning." —John 15:26-27 (New Jerusalem Bible translation.)

When Pentecost[*] day came round, they had all met together, when suddenly there came from heaven a sound as of a violent wind which filled the entire house in which they were sitting; and there appeared to them tongues as of fire; these separated and came to rest on the head of each of them. They were all filled with the Holy Spirit and began to speak different languages as the Spirit gave them power to express themselves. Acts 2:1-4 (NJB)

* Shavuot (Weeks), one of the three festivals at which it was customary to make pilgrimage to Jerusalem. It was 50 days (i.e., 7 weeks, or a week of weeks) after Passover and was considered to commemorate the giving of the Law to Moses at Mount Sinai.

With the coming of the Holy Spirit the Church begins its mission, just as Jesus began the work of his mission when the Holy Spirit descended on him at his baptism by John in the Jordan River. (See Mark 1:9-15 and parallels.)