Some people keep Shavuot on one day, some on another. Are some of those people simply rejecting the plain words of God?

Shavuot is the Hebrew name for Pentecost or the Feast of Weeks. The Bible says to count 7 weeks (49 days) from the Sabbath after the Passover, and then celebrate Passover the next day (day 50). However, the text seems a little ambiguous. Which post-Passover Sabbath is meant? The first day of Unleavened Bread or the weekly Sabbath?