Rashi on Rosh Hashana 22 omud 2 says that the baytusim wanted the 1st day of sefirah and Shavuos to fall on Sunday because of the possuk which says ממחרתהשבת which they interpret literally, that the omer should be brought after shabbos as opposed to after Sunday. What's this got to do with what day Shavuos falls on?
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