Gradually it became customary to celebrate the feast 50 days after the Passover. This gave it additional meaning. Jewish scholars had calculated that when their ancestors left Egypt, they arrived at Mount Sinai approximately 50 days later. It was there that God gave them the Law and made a covenant with them: "I will be your God, and you will be my people."
Thus, Pentecost became a time for the Jewish people to celebrate the covenant that made them God's chosen people.
The word "pentecost" is Greek for 50th day.
- Little White Book, Diocese of Saginaw