I was asking this same question myself, and I have found the answer, and it is fascinating. In Leviticus 23:10-11 and 15-17, The Word of God explains the calculation for what we call Pentecost (the Greek word for 50) as counting 50 days from the Feast of First Fruits, which is the first day after the first Sabbath after Passover. However, contemporary Jewish practice is to celebrate this Pentecost(called the Feast of Weeks in Hebrew, not called Pentecost) 50 days after Passover. In fact, this year, if you count 50 days from the day after the first Sabbath after Passover, you go past June 9, the date published on the Hebrew calendar, and end up on June 16. June 9 is 50 days from Passover. Dr. Thomas McCall, a theologian, wrote an article about this with an explanation about the descrepency, and you can access it at http://www.levitt.com/essays/pentecost.html. I hope this helps you.