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Second day of Shavous

I understand the basis for celebrating two days for all yamim tovim outside of the land of Israel, and for celebrating two days for Rosh haShana even within the land, but what is the basis for including Shavuot within that formulation? We commemorate two days for the first yontef of Pesach, on the possibility that that we live in a place beyond the timely reach of the bet din's emissaries (I live in Australia; it's guaranteed), but there are forty-nine days between that and Shavuot, which is more than enough time for people to know exactly which of those yamim tovim was the correct one. Since Shavuot is calculated on the basis of its being the fiftieth day after that first yontef, why the need to celebrate the fifty-first as well?

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