Pentecost is a Christian holy day that celebrates the coming of the Holy Spirit 40 days after Easter. Some Christian denominations consider it the birthday of the Christian church and celebrate it as such.
Originally, Pentecost was a Jewish holiday held 50 days after Passover. One of three major feasts during the Jewish year, it celebrated Thanksgiving for harvested crops. However, Pentecost for Christians means something far different.
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. John 14:16–18
And 40 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.