Question: Can you explain the day of Pentecost?
Answer: Our good question seems to refer to Acts 2: 1, "And when the day of Pentecost was fully come." There were three annual feasts of the Jews, the Passover, Pentecost, and the Feast of Tabernacles. Pentecost celebrated the beginning of their harvest. Pentecost (term found only in the New Testament, called "Feast of Harvest," etc. Ex. 23: 16) came fifty days after the Passover; hence, pente, five or fifty.
The apostles had been waiting for Pentecost. Jesus had told his apostles, "And, behold, I send the promise of my Father upon you: but tarry ye in the city of Jerusalem, until ye be endued with power from on high" (Luke 24: 49). We read in Acts 1: 4 that they are in Jerusalem waiting as Jesus had told them to do. Hence, the language "day of Pentecost was fully come" is significant.
The day of Pentecost is often called the hub of the Bible. The day of Pentecost in Acts 2 fulfilled many kingdom and salvation prophecies (cp. Joel 2: 28-32; Isa. 2: 2, 3, see Acts 2: 16-47). The apostles were baptized in the Holy Spirit to enable them to execute their commission, the full gospel was officially preached and Jesus' last will and testament came into being, the church become a reality, and remission of sin based on Jesus' shed blood was first experienced on the day of Pentecost.
The day of Pentecost was the birth of Christianity. It was on this memorable day that repentance and baptism in Jesus' name for the remission of sins was first taught and experienced (Acts 2: 38). The height of Joel's prophesy had been "whosoever shall call on the name of the Lord shall be delivered" (Joel 2: 32). When those whose hearts had been pricked asked what must they do, were told, and did it, they were calling on the name of the Lord (Acts 2: 37-42). People today are thus saved in the same way by calling on Jesus' name (Rom. 10: 13).