The word "Pentecost" comes from the Greek and translates as "fiftieth": it was the fiftieth day after the Israelite feast of "firstfruits", as follows:
First month of the Israelite year (Nisan or Abib):
Day 14 - Feast of Passover (Leviticus 23:5, Exodus 12:1-14) : the Israelites celebrated the delivery from bondage in Egypt, but one day at that feast, the death of the Lord Jesus brought to the believer deliverance from bondage to sin as our Passover Lamb (1 Corinthians 5:7).
Day 15 to 21 - Feast of Unleavened Bread (Leviticus 23:6-8, Exodus12:15-20) roast lamb and unleavened bread to give the Israelites strength for the journey, whereas the believer feeds on Christ and puts away evil (leaven) from his life (1 Corinthians 5:7-8).
Day 17 - Feast of First Fruits (Leviticus 23: 9-14) - the Israelites offered the first fruits of the harvest, so that God might be satisfied with them; the Lord Jesus rose from the dead three days and three nights later, giving proof of God’s satisfaction for His work of redemption (1 Corinthians 15:20,23).
Third month (Sivan):
Day 6 - Feast of Pentecost (Leviticus 23:15-21; Deuteronomy 16: 9) - seven weeks later the Israelites made ??offerings in anticipation of the wheat harvest; that year the descent of the Holy Spirit upon the gathered disciples, as Christ had promised, gave birth to His church, anticipating "much fruit" (John 12:24) from the "seed" which had died (Acts 2:1, Ephesians 2:16-18).
All the disciples gathered together in Jerusalem: "when the Day of Pentecost had fully come, they were all with one accord in one place waiting for the baptism of the Holy Spirit", in obedience to the Lord's instructions (verses 4-5).
In fulfilment of the promise of the Lord Jesus, the Holy Spirit descended on that day on these disciples, audibly, sounding like a very strong wind. Then they saw “a fire-like appearance presenting itself at first, as it were, in a single body, and then suddenly parted in this direction and that; so that a portion of it rested on each of those present.” (Hackett). Wind and fire are just symbols of the Holy Spirit, as well as oil (John 3:34), water (John 7:38-39), dove (Matthew 3:16), seal (Ephesians 1:13, 4:30) and pledge (Ephesians 1:14). The noise filled the whole house where they were sitting: as in a baptism, all who were there were immersed in it.
The 'appearance as of fire' in this opportunity should not be confused with the "baptism of fire". Baptism in the Spirit and the baptism in fire are mentioned together on one occasion (Matthew 3:11-12 and Luke 3:16-17), but they are two very different events: the first is a blessing, a privilege given to believers (the "wheat"), the second is a curse reserved as punishment for unbelievers (the "straw"). The baptism in the Holy Spirit of that pioneering group of disciples had them permanently inhabited by Him, receiving His power, and initiating the church of Christ. Before baptism, the Spirit of God had been with the disciples, just as He had been with the prophets and other men as reported in the Old Testament, but His residence with these was not permanent (Psalm 51:11).
From this occasion, while the Gospel was preached only to the Jews, the Holy Spirit was given to those upon whom the disciples put their hands (Acts 8:17, 9:17, etc.). When Peter opened the door of the kingdom of God to the Gentiles (Acts 10), the Holy Spirit, without delay and no other condition than simple faith, was permanently granted to all who believe (John 14:16, Acts 10:44, 11: 15-18). This condition prevails until today, and will remain until the church is raptured before the second coming of Christ. Every believer is born of the Holy Spirit (John 3:3,6, 1 John 5:1), Who then inhabits him making His temple of his body (1 Corinthians 6:19, Romans 8: 9-15, 1 John 2:27, Galatians 4:6), and is baptized by the Spirit (1 Corinthians 12:12-13, 1 John 2:20,27) and thus sealed to God (Ephesians 1:13, 4:30).
The New Testament distinguishes between "having" the Holy Spirit, which all believers have infallibly, and "filling" of the Holy Spirit, which is the privilege and duty of every believer, but requires his sanctification and obedience to God. A believer will sin against the Holy Spirit if he lies to his neighbour, if he is angry for a long time giving place to the devil, if he steals and does not work, if he is given to filthy talk, etc.; he will grieve the Spirit with bitterness, wrath, anger, clamour, evil speaking and malice (Ephesians 4:26-31); will quench or stifle the Spirit by doing evil, and despising the biblical teaching (1 Thessalonians 5:19).
At Pentecost, the disciples were not only baptized with the Holy Spirit, but also were "filled" with Him. This does not happen automatically, because we have the commandment "be filled with the Spirit" (Ephesians 5:18). It is also useless to pray that He will fill us, for it depends on our own sanctification. The fire-like tongues on that occasion are interpreted by many as a visible representation of a special gift that they were getting - that of speaking in foreign languages, as what is reported to have happened immediately after. Fire can be a type of the Holy Spirit, the source of this gift, for this was a figure of God to the people of Israel (Exodus 3:2, 24:17, Deuteronomy 4:24). It can also refer to the enthusiastic ardour with which the Gospel was then proclaimed.
The gift that was given to the disciples was to speak in a language that they had not learned, but were languages used at that time and were understood by the people from the various countries that spoke them, there present. The Lord Jesus had expressed His intention to make the Gospel to be preached throughout the world, and on this occasion, various languages ??spoken around the world were represented. One might think that this was how the message would be proclaimed throughout the world, but it was not so, as future events proved.
This was the third miracle, coming after the sound and tongues. We must not confuse these miraculous signs with the Holy Spirit Himself. No doubt, it was then that He came upon the disciples, but He is an invisible spirit and cannot be recognized by our physical senses. The signs were only evidence that He in fact had come to perform His mission in Christ's church.
The gift of tongues was also given to the house of Cornelius in Caesarea (Acts 10:44-47, 11: 15-17), to the disciples of John at Ephesus (Acts 19:6) and to the disciples in Corinth (1 Corinthians 14:1-33). Tongues was not a permanent gift, and Paul explains that it was a sign for unbelievers, and it should not be used unless there was someone present who could understand and interpret what was being said (1 Corinthians 14:22). The so-called "gift of tongues" today is nothing but jargon and hysteria.
Luke tells us, “there were dwelling in Jerusalem Jews, devout men, from every nation under heaven." What better occasion could there be to communicate the good news of salvation? The gift of tongues was ideal for this circumstance, and the Holy Spirit, God's wisdom, gave it to the disciples who were the messengers. Attracted by the noise, a large crowd gathered around the place where the disciples speaking in other languages were. Each one who came heard these simple Galileans speaking in their own language in which he was born. There was therefore much confusion, and they were all amazed and marvelled.
The disciples spoke of the greatness of God in these foreign languages - this would lead to praise, very appropriate in that feast in advance of harvest. However, the people were being prepared to hear about the greatest work of God in all ages: the ransom which had been paid for all sinners. This news would come following this, through the speech of Peter.
1 When the Day of Pentecost had fully come, they were all with one accord in one place.
2 And suddenly there came a sound from heaven, as of a rushing mighty wind, and it filled the whole house where they were sitting.
3 Then there appeared to them divided tongues, as of fire, and one sat upon each of them.
4 And they were all filled with the Holy Spirit and began to speak with other tongues, as the Spirit gave them utterance.
5 And there were dwelling in Jerusalem Jews, devout men, from every nation under heaven.
6 And when this sound occurred, the multitude came together, and were confused, because everyone heard them speak in his own language.
Act 2:7 Then they were all amazed and marveled, saying to one another, "Look, are not all these who speak Galileans?
8 And how is it that we hear, each in our own language in which we were born?
9 Parthians and Medes and Elamites, those dwelling in Mesopotamia, Judea and Cappadocia, Pontus and Asia,
Act 2:10 Phrygia and Pamphylia, Egypt and the parts of Libya adjoining Cyrene, visitors from Rome, both Jews and proselytes,
11 Cretans and Arabs—we hear them speaking in our own tongues the wonderful works of God."
12 So they were all amazed and perplexed, saying to one another, "Whatever could this mean?"
Book of Acts, chapter 2, verses 1 to 12