Catechism of the Catholic Church §2623 — excerpt
On the day of Pentecost, the Spirit of the Promise was poured out on the disciples, gathered "together in one place." While awaiting the Spirit, "all these with one accord devoted themselves to prayer."
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.