In Christion church, the date of Pentecost Sunday is 50 days after Easter, and in early days, many people were baptized in Pentecost Sunday, as people being baptized were given white robes to wear, thus it is also known as "whitsunday" or "White Sunday".
Pentecost is from the Greek word 'Pentekostos', which means 'fifty'. It's the 50th day after the Sabbath of Passover week and in Judaism is called the Feast of Weeks (Exodus 23, Exodus 24, Leviticus 16, Numbers 28, and Deuteronomy 16).
In John 14:16-18 Jesus tells His disciples that the Holy Spirit would come after Him. In the second chapter of the Book of Acts, we know that 40 days after Jesus’ death and resurrection, and 10 days after Jesus ascended into heaven, the promise of the Holy Spirit came to be - The Coming of the Holy Spirit.
And I will ask the Father, and he will give you another advocate to help you and be with you forever— 17 the Spirit of truth. The world cannot accept him, because it neither sees him nor knows him. But you know him, for he lives with you and will be in you. I will not leave you as orphans; I will come to you. John 14:16-18
After Jesus ascended into heaven, Jesus’ followers were gathered together for the Pentecost, and the Holy Spirit “filled the whole house where they were sitting” (Acts 2:2). “All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them” (Acts 2:4). This strange occurrence drew a large crowd, and Peter stood up to speak to them about repentance and the gospel of Christ (Acts 2:14). By the end of the day that the Holy Spirit came, the Church grew by 3,000 people (Acts 2:41).