In the Mishna Daf 17 there is an argument between Beis Shammai and Beis Hillel with regard to when Tashlumin are brought - depending on when Shavous falls. Beis Hillel concede that when it falls on a Shabbos the korbon is brought on the Sunday, however, because of the tzedokim the Kohen Gadol changes his uniform, hespedim are said etc to demonstrate that we are not following the wrong ruling of the tzedokim and keeping Yom Tov as from the start date of Sefiroh - the day after Shabbos.
Why was it necessary to make such drastic changes, no bigdei kehuna etc etc, surely the Beis Din who has the power to extend or shorten a month (when they were m'kadesh alpi adim) to ensure that Shavous would not fall on a Shabbos?
Looking forward to your answer.
Sholem
The Kohen Gadol wore Bigdei Kehunah. Rashi writes that he didn't wear Yom Tov clothes in the marketplace.
Regarding Kidush ha'Chodesh, at the time, the month was determined by the testimony of witnesses, not Beis Din's calculations. It is true that they sometimes threatened witnesses to make them testify or frighten them away (Rosh Hashanah 20a). However, that was put to very limited use due to the Chilul Hash-m it caused (ibid.).
Best wishes,
Mordecai Kornfeld
Kollel Iyun Hadaf