If you go back and read the Old Testament, you will discover that Pentecost was one of the Jewish feast days. Only they didn’t call it Pentecost. That’s the Greek name. The Jews called it the Feast of Harvest or the Feast of Weeks. When the day of Pentecost arrived, they were all together in one place. And suddenly there came from heaven a sound like a mighty rushing wind, and it filled the entire house where they were sitting. And divided tongues as of fire appeared to them and rested on each one of them. And they were all filled with the Holy Spirit and began to speak in other tongues as the Spirit gave them utterance. Now there were dwelling in Jerusalem Jews, devout men from every nation under heaven. And at this sound the multitude came together, and they were bewildered, because each one was hearing them speak in his own language. And they were amazed and astonished, saying, “Are not all these who are speaking Galileans? And how is it that we hear, each of us in his own native language? Parthians and Medes and Elamites and residents of Mesopotamia, Judea and Cappadocia, Pontus and Asia, Phrygia and Pamphylia, Egypt and the parts of Libya belonging to Cyrene, and visitors from Rome, both Jews and proselytes, Cretans and Arabians–we hear them telling in our own tongues the mighty works of God.” And all were amazed and perplexed, saying to one another, “What does this mean?” But others mocking said, “They are filled with new wine.” (Acts 2:1-13). Pentecost was the celebration of the beginning of the early weeks of harvest. In Palestine, there were two harvests each year. The early harvest came during the months of May and June; the final harvest came in the Fall. Pentecost was the celebration of the beginning of the early wheat harvest, which meant that Pentecost always fell sometime during the middle of the month of May or sometimes in early June. There were several festivals, celebrations, or observances that took place before Pentecost. There was Passover, there was Unleavened Bread, and there was the Feast of Firstfruits. The Feast of Firstfruits was the celebration of the beginning of the barley harvest. Here’s the way you figured out the date of Pentecost. According to the Old Testament, you would go to the day of the celebration of Firstfruits, and beginning with that day, you would count off 50 days. The fiftieth day would be the Day of Pentecost. So Firstfruits is the beginning of the barley harvest and Pentecost is the celebration of the beginning of the wheat harvest. Since it was always 50 days after Firstfruits, and since 50 days equals seven weeks, it always came a “week of weeks” later. Therefore, they either called it the Feast of Harvest or the Feast of Weeks. Modern Christians observe Pentecost as a holiday, not to celebrate a wheat harvest, but to remember when the Holy Spirit invaded the Church in Acts 2. On Pentecost Sunday, the Holy Spirit filled the Church with power and added 3,000 new believers. The account in Act 2 reports that, after Jesus ascended into heaven, Jesus’ followers were gathered together for the Feast of Harvest (aka Pentecost), and the Holy Spirit “filled the whole house where they were sitting” (Acts 2:2). “All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them” (Acts 2:4). This strange occurrence drew a large crowd, and Peter stood up to speak to them about repentance and the gospel of Christ (Acts 2:14). By the end of the day that the Holy Spirit came, the Church grew by 3,000 people (Acts 2:41). This is why Christians still celebrate Pentecost. The Holy Spirit was prophesied in the Old Testament and promised by Jesus.  Jesus promised the Holy Spirit in John 14:26, who would be the Helper for his people. “But the Helper, the Holy Spirit, whom the Father will send in My name, He will teach you all things, and bring to your remembrance all that I said to you.” This New Testament event is also significant because it fulfills an Old Testament prophecy in Joel 2:28-29: “And afterward, I will pour out my Spirit on all people. Your sons and daughters will prophesy, your old men will dream dreams, your young men will see visions. Even on my servants, both men and women, I will pour out my Spirit in those days.”