"Pentecost is a Christian Holy Day that celebrates the coming of the Holy Spirit 40 days after Easter...
Before Jesus was crucified, He told His disciples that the Holy Spirit would come after Him. 'And I will ask the Father, and He will give you another Counselor to be with you forever – 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 '(Jn 14:16-18).
Forty days after Jesus was Resurrected (10 days after He Ascended into Heaven), that promise was fulfilled when Peter and the early Church were in Jerusalem for Pentecost, “When the day of Pentecost came, they were all together in one place. Suddenly a sound like the blowing of a violent wind came from Heaven and filled the whole house where they were sitting. They saw what seemed to be tongues of fire that separated and came to rest on each of them. All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them (Acts 2:1-4).”
--- Richard Wagner, Digitalwalk.net