The 11 remaining Apostles returned to Jerusalem after the Ascension of Jesus into Heaven. Acts 1:14 says, “All these with one accord devoted themselves to prayer, together with the women and Mary the mother of Jesus, and with his brethren.” During this time, the Apostles chose a replacement for Judas Iscariot. They cast lots between two men, and the lot fell on Matthias.
After nine days of prayer, the Holy Spirit descended upon the disciples. They heard a sound like a “mighty wind,” and “tongues as of fire” were resting on their heads.
Going out, Peter to the crowd that had gathered. Even though, just a few weeks earlier, he had denied Christ three times, Peter courageously proclaimed the Gospel. That day, three thousand people believed in Christ and were received into His Church.