According to mainline Protestant theology, why is Pentecost on Sunday and why did it lose its significance?

**Christian Pentecost** It is well known that the early Christians were Jews who accepted Jesus, who was also a Jew, as the messiah. These Christian Jews also observed Shavuot, a Jewish holiday but later changed the name to Pentecost (its Latin name). The New Testament Book of Acts 2 explains that the Holy Spirit descended upon the eleven apostles (since the twelfth, Judas', passing). Thus Pentecost is “the birthday of the church.” Interestingly enough, this is similar to the modern Jewish concept of Shavuot which commemorates the revelation of the Torah. Of course, the original meaning of Shavuot was not to celebrate the giving of the Torah at Sinai since the Torah never selected a date for the Sinai revelation. Nevertheless, this was the meaning or how Shavuot was practiced (and still is) during Second Temple times (ie the times of Jesus). Leaving aside the fact that Shabbat was originally on Sunday and that the rabbis later changed it to Saturday and Christians back to Sunday, why don't Christians (who consider themselves to be spiritually chosen, in a sense, nevertheless, keep Pentecost on Saturday, “Sabbath” since this was the date chosen by the rabbis? And why don't Christian also celebrate it as the date of the giving of the Torah? Christians also have a tradition that the Apostles were dressed in white, thus calling Pentecost Whitsunday (white Sunday).