Why do we celebrate the second day of Shavout in Chutz Laaretz? - when we can already know when it will be because it not hard to count 50 days from second day of Pesach and by then all of the Jewish world know when Pesach was and is able to calculate when Shavuot will be with out the need of second day Shavout.

The Talmud (Beitza 4b) comments on the fact that today we know the setting of the calendar and there is no need for second day Yom-Tov, the Talmud answers that we should keep our forefathers custom since it may happen that the nations will decree against Torah study, we might forget the calendar settings, loose track of time and G-d forbid come to eat Chametz on Pesach (Rashi ibbid) The Chatam Sofer (OC 145, YD 250) asks your question, he answers that we should keep a standard practice for all festivals so we don't come to cheapen the other Yamim Tovim. One should add that from Tshuvot Hageonim (116:29) it appears that there is another reason, two days Yom Tov outside of Eretz Yisrael were planned from the outset by Hashem and Shavuot is the proof for that. From the Magen Avraham (OC 494, 1) it seems that second day of Shavuot fits with the fact that Torah was given on the 51st day of the Omer…