Pentecost

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.

1. 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.

“Through this baptism of the Holy Ghost and fire, the apostles became more knowing, and had a greater understanding of the mysteries of the Gospel, and were more qualified to preach it to people of all nations and languages.”

2. 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.”