Jesus promised He would send the Holy Spirit after He ascended, and so about a week after He ascended, He kept His promise and the Holy Spirit was poured out upon all those gathered in the upper room. When this happened, they began to speak different languages. This happened during the Jewish Feast of Shavu'ot, also called Pentecost. Shavu'ot was one of the three major festival on the Jewish calendar, where God commanded all Jewish males to come to the Tabernacle (later, the Temple in Jerusalem) to offer sacrifices. Because of this, Jerusalem was full of Jewish pilgrims from many different places, who all spoke different languages. They took notice of the twenty or so people who started speaking, because they were speaking the languages of these different Jewish pilgrims. Some among the pilgrims were astonished, "Are these not Galileans, and yet they are speaking in our languages?" but others scoffed and said, "Pfft, these guys are drunk as heck".
This is when St. Peter, full of the Holy Spirit, stood up and addressed the crowd that was gathering. He then gave a lengthy sermon, and quoted the Prophet Joel. Joel had prophesied that in the last days God would pour out His Spirit on all flesh, "your sons and daughters will prophesy, your young men will see visions, your old men will dream dreams", then Peter says that Joel's prophecy was being fulfilled right then and there. This was the fulfillment of Joel's prophecy, on Pentecost two thousand years ago, when God poured out the Holy Spirit upon His Church. Peter then went on to preach that Jesus was the long awaited Messiah, and though Jesus was put to death by the plotting of wicked men, God raised Him up, vindicating Him, and that He is truly Lord Messiah, and now is the time to repent, believe the good news of God's victory through the Messiah, and to come and be part of what God is doing through Jesus by receiving baptism and repenting.
When the crowd heard these words, they believed and they received baptism and became Christians, about three thousand in total. And that is the day as Christians we usually remember as the "birthday" of the Church. When God fulfilled His promise to send the Holy Spirit, and the Apostles were given the power to bear witness to Christ beginning in Jerusalem, throughout the cities of Judea and Samaria, and then further to the various nations of the world. Soon the Apostles would go from preaching in Jerusalem and Judea, and would be preaching the Gospel in places like Antioch, Ephesus, Athens, and even the heart of the Roman Empire, the city of Rome itself. This is the day when God inaugurated His Church to the Gospel mission which she has been living for the last two thousand years, which we are still living today--by the power of the Holy Spirit who was poured out two thousand years ago on Pentecost.
-CryptoLutheran