The celebration of Pentecost (this year, May 31) commemorates the meeting of Jesus’ apostles in which there came a sound from heaven as a rushing mighty wind and cloven tongues of fire that sat upon each of them. It happened on the Jewish festival day Shavuot, the Feast of the Harvest or the Feast of Weeks.
BYU professor Eric Huntsman shares this history along with ideas for celebrating Pentecost on his blog, Latter-day Saint Seasonal Materials. He explains that this joyous spiritual endowment caused Peter to become bold in preaching.
Richard Neitzel Holzapfel, an Area Seventy, explains that as Peter preached boldly and publicly at the temple to Jews gathered for the festival, Peter initiated the apostolic mission to preach Christ in all the world. See Richard Neitzel Holzapfel, A Lively Hope: The Suffering, Death, Resurrection, and Exaltation of Jesus Christ, especially the chapter “The Exaltation of Jesus Christ on the Right Hand of God.”
In the realm of the Book of Mormon, some scholars suggest that Abinadi delivered his speech to King Noah (Mosiah 11-12) because of many parallels between the festival Shavuot and Abinadi’s message. Shavuot is a celebration of God’s Law, and Abinadi’s message focused on how King Noah had failed in keeping the Law. See the chart and readings at Book of Mormon Central, Knowwhy 90.