If you go back and read the Old Testament, you will discover that Pentecost was one of the Jewish feast days. Only they didn't call it Pentecost. That's the Greek name. The Jews called it the Feast of Harvest or the Feast of Weeks. It is mentioned in five places in the first five books — in Exodus 23, Exodus 24, Leviticus 16, Numbers 28, and Deuteronomy 16.