After His resurrection Jesus appeared over a period of forty days and spoke about the kingdom of God - this understanding about the kingdom of God must be extremely important!

As we enter into the New Testament book of Acts, it becomes very obvious how important the role of the Holy Spirit is in the life of a Believer. Once again, we can see the fulfillment of the prophecies of the Old Testament being fulfilled in the New Testament. Pentecost (Feast of Weeks) was celebrated in the Old Testament as a praise and thanks to God for the barley harvest. The Holy Spirit came to dwell within the Believers at Pentecost to begin the new harvest. Jesus was very clear that the apostles were to wait in Jerusalem until they had been baptized with the Holy Spirit. For new Believers prior to that time, the Holy Spirit came upon them at the moment of conversion.  At Pentecost, the Holy Spirit came to dwell within.

Pentecost was observed on the 50th day, or 7 weeks, plus one day past the Saturday Sabbath that occurs during the Feast of Unleavened Bread. The Old Testament describes this celebration as The Feast of Weeks. In the New Testament it is called Pentecost. (Leviticus 23:15-21)

At Pentecost, or the Feast of Weeks, the ripe barley was presented as a thanksgiving offering before the Lord. This was done early in the morning of that day by 9:00 am., which incidentally was around the same time that the Holy Spirit came to those believers in Jesus.

Acts 2:1-13 records the coming of the Holy Spirit at Pentecost.
Acts 2:4
All of them were filled with the Holy Spirit and began to speak in tongues.

Not only were the apostles given the gift of speaking in tongues (other languages) but were given ALL the spiritual gifts.  This becomes evident as we continue to read about the things they did and said in the rest of the New Testament.

Pentecost was one of the three commanded holy days. So at this time God-fearing Jews had come to Jerusalem from different nations and spoke different languages.  Even though the apostles received all the spiritual gifts the most urgent immediate need was the gift of speaking in other languages. They were given this gift so that they could witness to all the people gathered together for Pentecost. Those people then returned to their own homes with knowledge of Jesus, spoken to them in their native language.

By the power of the Holy Spirit the disciples could speak and understand these national languages having had no training in any of them.

Paul writes in some of his letters about the spiritual gifts and mentions speaking in tongues or in an unknown language.  He also says that for a person to speak in tongues publically, there is a need for an interpreter of tongues to be present.

1 Corinthians 14:2, 28
If you speak languages that others don't know, God will understand what you are saying, though no one else will know what you mean. You will be talking about mysteries that only the Spirit understands.  If no one can explain, you must keep silent in church and speak only to yourself and to God. 

When the apostles spoke in the different national languages, they were speaking to people who were able to understand them.  Since the apostles were given all the spiritual gifts they were all able to understand and interpret all the languages that were spoken.  Other verses that may be helpful in understanding are Matthew 6:6 and Romans 8:26 among many others. 

Peter begins to teach the people from the prophecies of Joel and clearly points to the return of Jesus. Very soon after Jesus ascended into heaven, His followers were looking for His return.

"About three thousand were added to their number that day". They were baptized but it says nothing about water. They needed to be baptized with the Holy Spirit. However, there were immersion pools that the Jews used for ceremonial cleansing.  Interestingly this number three thousand is found in another place in Scripture.  It was during the "golden calf" incident.

Exodus 32:28
And the sons of Levi did according to the word of Moses. And there fell of the people that day about three thousand men. 

Three thousand unbelievers perished that day and three thousand believers were saved at Pentecost.