Why did they speak in tongues on the day of Pentecost? (Shavuot)

On the day of Pentecost, there were Jews from every nation under heaven in Jerusalem. As commanded by God, they were there for the Feast of Weeks (Shavuot). They were there to appear before the Lord and give an offering at the Temple. (Deuteronomy16:10, 16)

These Jews all became witnesses of the amazing fulfillment of the prophecy of the prophet Joel: God had begun to pour out of His Spirit on all flesh! (Acts 2:16-17)

The Jews marveled when they heard the hundred and twenty disciples speaking in the tongue of the countries they were born. These Galilaeans didn’t understand the words they were uttering, but the Jews that were present from around the world did. The disciples were speaking with other tongues as the Spirit gave them utterance. The Jews heard them speaking in their languages the wonderful works of God! (Acts 2:4-11)

God spoke through them as they allowed God to change their words into another language. (Acts 2:8)
This was the beginning of tongues as the sign of receiving the gift of the Holy Ghost. (Acts 2:4, Acts 10:45-46, Acts 11:15, Acts 19:6) God’s promise had begun to be fulfilled. (Acts 1:4, Acts 2:33, Acts 2:39)

In what language was the gospel preached on the day of Pentecost?

The Jews that were gathered in Jerusalem that day not only knew their native language from the country they were from, but as Jews, they also knew the Hebrew language. They most likely knew Greek as well, as it appears to have been the universal language at that time. Because all those that were gathered were devout Jews, it is most likely Peter was preaching in the Hebrew language. (Acts 2:5)

The gospel message on the day of Pentecost was preached by Peter, not by the hundred and twenty speaking in tongues. The Bible clearly records the preaching that day, and it was by Peter. (Acts 2:14-40)

"But Peter, standing up with the eleven, lifted up his voice, and said unto them, Ye men of Judaea, and all ye that dwell at Jerusalem, be this known unto you, and hearken to my words:" - Acts 2:14

The conclusion of his gospel message: what we must do to obey the gospel.

"Now when they heard this, they were pricked in their heart, and said unto Peter and to the rest of the apostles, Men and brethren, what shall we do?" - Acts 2:37

"Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost." - Acts 2:38

"For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call." - Acts 2:39