Pentecost is the day the Holy Spirit came upon the first Christians. Each year many people celebrate the day of Pentecost. It occurs 50 days after Passover every year.
The first Christians gathered in Jerusalem on the day of Pentecost (Acts 2:1-5). The Holy Spirit came upon them with what appeared to be like tongues of fire, and they spoke in various tongues.
The tongues they spoke were real languages on earth. "And when this sound occurred, the crowd came together, and were bewildered because each one of them was hearing them speak in his own language." (Acts 2:6)
However, this is not the way God the Holy Spirit comes upon believers anymore. We are not required to gather in Jerusalem like the first Christians were.
And we receive the Holy Spirit when we believe instead of waiting at a specific place (Jerusalem) for a specific time (day of Pentecost).
The Bible tells us:
“And you also were included in Christ when you heard the message of truth, the gospel of your salvation. When you believed, you were marked in Him with a seal, the promised Holy Spirit, who is a deposit guaranteeing our inheritance until the redemption of those who are God’s possession.” (Ephesians 1:13-14)
This verse tells us that the Holy Spirit lives in us when we believe in Jesus.
The Bible also tells us that while speaking in tongues in the early days was a more common gift, it is not the greatest gift. It is also not a gift that is given to every Christian.
"And God has appointed in the church, first apostles, second prophets, third teachers, then miracles, then gifts of healings, helps, administrations, various kinds of tongues. All are not apostles, are they? All are not prophets, are they? All are not teachers, are they? All are not workers of miracles, are they? All do not have gifts of healings, do they? All do not speak with tongues, do they? All do not interpret, do they? But earnestly desire the greater gifts." (1 Corinthians 12:28-31)
The Bible tells us to earnestly desire the greater gifts. Speaking in tongues is actually the least of the gifts (especially now, because there are Christians and translators in almost every nation and tribe).
See also Pentecostal and Charismatic teachings.