The Catechism in Questions & Answers

520. When was the promise of the outpouring of the Holy Spirit fulfilled?

It was on Pentecost in Jerusalem that this promise was fulfilled when the Holy Spirit was poured out upon the Apostles and the disciples.

“When the day of Pentecost had fully come, they were all with one accord in one place. And suddenly there came a sound from heaven, as of a rushing mighty wind, and it filled the whole house where they were sitting. Then there appeared to them divided tongues, as of fire, and one sat upon each of them. And they were all filled with the Holy Spirit and began to speak with other tongues, as the Spirit gave them utterance.”
Acts 2: 1-4