In the Old Testament, the Feast caled Pentecost, or the Feast of Weeks, celebrated the beginning of the wheat harvest, then later, the giving of the Law to the People of Israel. It celebrated God's past work and foreshadowed and celebrate a coming greater blessing. It is no wonder that the blessed coming of the Holy Spirit, as promised by Christ, came 50 days after Passover, the day of his resurrection, on Pentecos, anticipating a new more glorious coming of the Spirit. As Jesus prepared to ascend, he promised his followers that the Spirit would come to empower the gospel's promulgation to the Jews, Samaritans and then to usher in the kingdom of God to include the Gentiles. (Act 1:8) Ten days after the ascension, while they were in Jerusalem celebrating the Pentecost, all Jews, who could, were expected to pilgrimage to the temple for the event, anticipating the wheat harvest and the giving of the Law, and a greater blessing from God in the consummation of his kingdom. It was on that day that the Spirit fell on "devout men from every nation" (Acts 2:5). "Devout men" would have been a reference to the Jews scattered throughout the empire as a result of the diaspora. 

The Old Testament ends with some of the Jews repatriating to Palestine, while others remained in exile. The Bible speaks diminutively of the Jews who remained, yet clearly shows God's grace and care for them. The book of Ezra speaks of the caravans of God's people returning to Palestine. The people came out in several waves of returning refugees. The most famous of their leaders were Shealtiel, Zerubbabel, and Ezra each of whom restored a different aspect of government, culture and religious tradition to the repatriated Jews. Afterward the nation was re-established a royal official in the court of Medo-Persia, Nehemiah, returned to restore the Holy City of Jerusalem.  Ezra and Nehemiah record how God's fulfilled his promise to return his people to the land and to the practice of the Mosaic Law. Yet, some Jews chose not to return and failed to participate or benefit from the blessing of the restoration. They remained in expatriated around the Lavant and in what at the time of first century Pentecost was the Pax Romana. Most Judeans, who had retuned in Exile, were patronizing toward them. God's work among them seemed to almost ancillary to those who remained in the diaspora. The book of Esther records God deliverance of the Jews in the diaspora. God is never directly named or referenced, nor does he ever speak. God remained faithful to his covenant people even to those who had not been willing to participate in all God had for them. Separated from the national promises, they were the lesser of the two groups of Jews. Jesus' ministry had been contained to within walking distance of Judea. 

As Jews gathered in first century Jerusalem on Pentecost, the Holy Spirit came in the form of tongues of fire. As He fell on the twelve, then the assembled Judean, disparate, Jews showing that the deliverance to his people was now complete. A new period of anticipated blessing commenced. As those Jews from the exiled nations spoke in tongues, the division between Israel and Judah was reconciled the Judean and diasporate Jews experienced the very same blessing of of the Spirit, the reception of tongues. Yet, the blessings went further. The covenants given to Abraham and David continued and brought a much broader fulfillment even of this baptism of the Spirit, evidenced in hearing the gospel in distinct languages of listeners (Acts 2:6-11). This Pentecost event was foreshadowing a new period where Judean Jews, Hellenistic Jews (those dispersed throughout the Roman Empire) and were all brought into a new covenant. Leading those present that day to ask; "What does this mean?" (Act 2:12) All who assembled that day understood that this activity of the Holy Spirit was what Jesus had told them to anticipate. A promise that Jesus had said would be greater than his own redemptive work and would enable them to do even more than he had done and would bring peace to all who lived in that blessing (John 13-17).

Peter under the influence of the Spirit explained that according to the Scripture this was the beginning of a new dispensation, whereby God's covenant would be realized among all people, without regard to ethnicity, gender or history. God's covenant of redemption was being continued and expanded, while aspects of the Old Covenant were no longer in play. At the end of his sermon many responded with a desire to join this new work of God:

[37] Now when they heard this they were cut to the heart, and said to Peter and the rest of the apostles, “Brothers, what shall we do?” [38] And Peter said to them, “Repent and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins, and you will receive the gift of the Holy Spirit. [39] For the promise is for you and for your children and for all who are far off, everyone whom the Lord our God calls to himself.” (Acts 2:37–39ESV)

Peter promise that when an individual received Christ, in the proscribed manner, then God would give to him the promise Holy Spirit as he had to those at Pentecost. Jesus and the twelve had been Judean and their ministry had been exclusive to the Judeans (Mt 15:24). The first part of the 28 Chapter Book of Acts (through chapter 15) describes how this promise is fulfilled in the life of the church, and how the Holy Spirit is received. Yet, since it is unclear whether, either Samaritans, who would not have been welcomed, or Gentiles, who had no reason to attend were present at that Pentecost. Many Jews believed that Gentile salvation would require adherence and adoption of Jewish traditions and practices. There are three additional incidences where tongues are manifested. Jesus and his followers had been thoroughly Jewish, continuing in the practices and feast of Jerusalem. They expected new believers to continue the same traditions. Originally, they showed little incentive to proselytize outside Judea but under the persecutions, they began dispersing and the gospel spread.  Three incidences in the book of Acts are critical to answer the question, what does this mean? 

1. In Acts 8 we encounter the first Samaritans who come to Christ. (Samaritans were a mixed-race group related to the Jews who had also mixed worship of Jehovah with idolatry). After hearing of their conversion Peter and John visited to observe what had taken place:

[14] Now when the apostles at Jerusalem heard that Samaria had received the word of God, they sent to them Peter and John, [15] who came down and prayed for them that they might receive the Holy Spirit, [16] for he had not yet fallen on any of them, but they had only been baptized in the name of the Lord Jesus. [17] Then they laid their hands on them, and they received the Holy Spirit. (Acts 8:14–17ESV) 

There apparently had been no instruction or understanding of the Holy Spirit in Phillips presentation of the gospel. The disciples laid hand on them and they "received the Holy Spirit." The Samaritans, though related to the Jews would not have been at Pentecost, they would have been ostracized. There is no doctrinal association with this, but the "reception of the Spirit" and "his falling on them" (which implies they spoke in tongues) shows the disciples that the Samaritans could receive the Holy Spirit, just as they did. They are returned and brought into the same experience of the Spirit as the Jews had. They are among the New Covenant believers, no different in status than the Jews. It is clear that they received the baptism after their conversion, just as the Jews had at Pentecost. 

 2. The second incident occurs at the first recorded conversion of a Gentile, Cornelius, a Roman soldier. The Apostle Peter received a vision of a blanket carrying foods that were unclean under the Law for him to eat and told to eat them. A vision that represented the abolishment of the Mosaic Law (Hebrews 8:13). The Law had restricted Gentiles from inclusion unless they became circumcised and abided by the Jewish Law. Peter is then told to go and visit the Roman Soldier Cornelius, who comes to Christ, and whom Peter baptizes with no requirement of conformity to the Law. 

[44] While Peter was still saying these things, the Holy Spirit fell on all who heard the word. [45] And the believers from among the circumcised who had come with Peter were amazed, because the gift of the Holy Spirit was poured out even on the Gentiles. [Italics mine] [46] For they were hearing them speaking in tongues and extolling God. Then Peter declared, [47] “Can anyone withhold water for baptizing these people, who have received the Holy Spirit just as we have?” [48] And he commanded them to be baptized in the name of Jesus Christ. Then they asked him to remain for some days. (Acts 10:44–48ESV)

To the amazement of Peter and the other Jews with him the Spirit fell on uncircumcised believers as He had on the circumcised. The Pentecostal blessing had come to people who had no regard for the Law. As John the Baptist had predicted with the coming of the covenant (violent - people hostile toward the Law) would enter into the New Covenant on their own (Luke 16:16). 

While some continuationists will accuse me of "reading something" into the text that isn't there. Peter explains this event in exactly the terms I have stated. When he returns to Jerusalem the propriety and authenticity of his actions are challenged. The Jewish leaders did not believe that unclean Gentiles could experience redemptions without become clean under the Jewish Law.  To them the idea was foreign to any paradigm they had imagined, but Peter defended his actions:

[15] As I began to speak, the Holy Spirit fell on them just as on us at the beginning. [16] And I remembered the word of the Lord, how he said, ‘John baptized with water, but you will be baptized with the Holy Spirit.’ [17] If then God gave the same gift to them as he gave to us when we believed in the Lord Jesus Christ, who was I that I could stand in God’s way?” [18] When they heard these things they fell silent. And they glorified God, saying, “Then to the Gentiles also God has granted repentance that leads to life.” (Acts 11:15–18 ESV)

Peter related "baptism of the Spirit," and manifestation of tongues to "that which had happened to them "at the beginning," meaning the beginning of the new era promise in Joel and initiated at Pentecost. But there is, yet, another incident.

3. In Acts Chapter 19 we read of an incident where the ministry of the evangelist, Apollos, intersects with the ministry of Paul when they are both in Ephesus. Apollos was a Jew, with a Greek name, probably from the Jewish community that had been in Egypt since many had fled there to escape Babylonian captivity. Apparently, Apollos prior to conversion had been a follower of the teachings of John the Baptist. John the Baptist had been a forerunner of Christ, the last of the Old Testament prophets, but he lived and taught according to the Law.  Apollos had heard about Christ, but in a time prior to the invention of the printing press, where all communication was by oral tradition, it is unclear, what Apollos, understood or preached concerning the gospel. While in Ephesus he encounters Paul's students Acquilla and Priscilla, and they instruct him on the significance of the Holy Spirit's work and coming, which was not part of John's teaching:

[1] And it happened that while Apollos was at Corinth, Paul passed through the inland country and came to Ephesus. There he found some disciples. [2] And he said to them, “Did you receive the Holy Spirit when you believed?” And they said, “No, we have not even heard that there is a Holy Spirit.” [3] And he said, “Into what then were you baptized?” They said, “Into John’s baptism.” [4] And Paul said, “John baptized with the baptism of repentance, telling the people to believe in the one who was to come after him, that is, Jesus.” [5] On hearing this, they were baptized in the name of the Lord Jesus. [6] And when Paul had laid his hands on them, the Holy Spirit came on them, and they began speaking in tongues and prophesying. (Acts 19:1–6 ESV)

Apollos represented a group of diasporate Jews who probably would not have taken the pilgrimage to Jerusalem at Pentecost, due to distance, who are brought into inclusion in its blessing.

What conclusions do we draw from these three incidences in answer to the question at the time (and still debated today) about what does Pentecost mean? Let me make some observations:

• Each incident where tongues accompany either salvation, baptism or a separate event, occurs at the first known post-first-century-Pentecost conversion of one of the non-Judean groups. The first occurrence is among Samaritans. An ethnic group related to the Jews but ostracized from them, this group was not mentioned as being present at the Pentecost event. The phrase to "devout men from every nation under heaven," (Acts 2:5ESV) is undoubtedly a reference to Hellenistic (or dispersed Jews) The second occurrence was the Gentile family of Cornelius.  it is unclear whether any gentiles were present at the first century Pentecost celebration, yet Peter proclaims that the event fulfills the promise made to Joel which is a promise of regeneration to all flesh. The third incident involves a believer who was preaching an incomplete or partial gospel, that was still rooted in law and the teaching of the John the Baptist and appears to clarify that the gospel includes the teaching of the Holy Spirit Baptism.
•  These three events are not normative in the book of Acts, no other conversion recorded is either accompanied by tongues nor is a distinction made between baptism of the Spirit and conversion.
• Each of these groups are brought into the kingdom without either being circumcised or following the law. 
• No where else in the New Testament is there a suggestion that all believers are expected to speak in tongues as a result of the Baptism of the Spirit. In first Corinthians 12: 30 Paul asks rhetorically, "Do all speak in tongues." The grammatical construction makes it evident the answer is "no." The New Testament makes it clear that upon reception of Jesus Christ one received all righteousness and fullness (Romans 8:9 & 10; Colossians 2:9) and is fully baptized into the body of Christ (1 Cor 12: 12 &30).
• However, the gift of tongues (and other miraculous gifts) function, they were expected to eventually wane away (1 Cor 13: 8-10; Hebrew 2:2-4)