1. Question: Why is one allowed to accept Yom Tov early on Shavuos, before sunset?
A. On Shavuos we make it a point to daven maariv and say Kiddush once it is completely dark in order so that 49 complete days have passed in the omer as the Torah says should happen before the onset of Shavuos.
B. Rav Nissim Karelitz said that if you take on yom tov early only regarding stopping doing melacha that does not take away from the 49 complete days needed.
Answer: Since that time is still considered the 49th day of the omer, it’s just that it’s forbidden to do melacha because of Tosefes yom tov, therefore it’s not really recognizable that you are impeding on the day rather you are just passively not working (See Dirshu Mishna Berura 494 footnote 2).