Q. B’H I live in Yerushalaim, but I visit my parents often during Yom Tov. They have asked me why I keep two days Shavuos, or why they should even do so, since after all we counted the Omer with a brocho and Hashem’s name, so we clearly know which day is going to be Yom Tov?

A. Horav Shlomo Miller Shlit”a opinion is that in reality we all know when the days of Yom Tov are and the counting of the Omer is a separate mitzva in itself not necessarily interdependent on when Shavuos should be.
(Similar answers are given by other Poskim, including Chassam Soffer).

Rabbi A. Bartfeld as revised by Horav Dovid Pam, Horav Aharon Miller, Horav Chanoch Ehrentreu and Horav Kalman Ochs Shlit’a.