Second day of Shavous
Shavuos is one Yom Tov that has nothing to do with the day of the month. It is 50 days after the first day of Pesach. Even in the times of making Rosh Chodesh via the sight of the new moon, by the time Shavuos rolled around everyone knew definatively when Pesach started. Why then do we keep 2 days Yom Tov on Shavuos?