The confusion on the timing of Shavuot is due to the fact that Leviticus 23:11 does not specify what it means by the word "Sabbath," whether the festival day (Lev. 23:7), or the weekly Sabbath (Lev. 23:3).  The Pharisee's taught that the count was to begin from the day after the festival Sabbath, while the Sadducees believed that the count was to been on the day after the weekly Sabbath. 

The following arguments demonstrate that the Pharisee's were correct, and that we should follow their example. 

1. Examination of the Exodus account shows that the Law was given on the 50th day after the Passover;  the Sadducean view makes the number of days between Passover and the giving of the Law variable, thus negating the purpose of the 50 days. 

2. Seven complete Sabbaths were to be counted between Passover and Shavuot.  The Sadducean interval contains 8 complete Sabbaths in 6 out of 7 years.  The significance of the counting of the Sabbaths is that the Law was given on the Sabbath day.  Putting this celebration on Sunday negates the reason for counting the seven Sabbaths. 

4.  The Resurrection Day of Yayshua the Messiah was on the "first of the Sabbaths," after Passover, that is, the first Sabbath counted to Shavuot.  But the first Sabbath after Passover is always skipped in the Sadducean count.  Thus, the Resurrection day disproves the Sunday Pentecost. 

5. Luke 6:1 speaks of the "second-first Sabbath," which is equivalent to the anniversary of the Resurrection day, and Matthew's "Latter of the Sabbaths."  The first Sabbath of unleavened bread is the Passover high day (John 19:31), but the weekly Sabbath of unleavened bread is also called "first" on account of Lev. 23:15-16.  To distinguish it from the high day, it is sometimes called the "second first Sabbath."  In the Sadducean view, the day after this Sabbath begins the count to Shavuot, but alas!, Luke records the disciples eating the grain in the fields, an act that was forbidden until the the count began (Lev. 23:11-14).  The disciples were eating new grain a day before the Sadducees believed it was lawful.  Thus with one stroke of the pen, Luke buries Rabbinic tradition and Sadducean heresy. 

6. Joshua 5:10-12 explains Lev. 23:11 as the "the morrow of the Passover" which is the 16th standard day of Aviv.  Furthermore, that year the Passover fell on the first day of the week, so that the morrow of the Seder was the second day of the week, not Sunday.  The text relates that they ate the new grain on that day, not on the Sunday a week later.  Therefore, Shavuot was counted from the second day of the week that year, which disproves the Sunday Pentecost theory. 

7. The Samaritans are the historically known originators of the Sunday Pentecost doctrine, and to Judaism, this is the oldest known heresy.  The Sadducees, who followed them, lacked spiritual credibility, in that they denied the resurrection of the dead. 

8.  Josephus, without exception says that Israel observed Shavuot according to the 16th of Aviv. 

9.  The Sadducees and Priests had to conduct all temple ritual's according to the Pharisees Segan in the Temple, appointed and empowered to ensure that all ritual's were conducted according to the Pharisees interpretation. 

10.  That the Sadducees controlled the temple is the hearsay of modern liberal scholarship influenced by the Church.  There is no historical evidence that they had any say in how the religious aspects of Torah were applied.