QUESTION: This year Shavuos falls out on Monday and Tuesday. May I cook and set the table for the Monday night meal on Monday afternoon?
ANSWER: The second day of Yom Tov is not Biblically ordained; on a Biblical level Shavuos lasts for one day. The second day of Yom Tov is a Rabbinic enactment. Just as it is forbidden to prepare on Yom Tov for a weekday, so too is it forbidden to prepare for the second day of Yom Tov on the first day, because on the first day the second day of Yom Tov is regarded as a weekday. The Gemara refers to this as hachanah (preparing). This category includes both melachos, such as slaughtering an animal or cooking, as well as actions which involve tircha, extra work, such as washing dishes (Shulchan Aruch and Mishnah Berurah 503:1), setting a table (Rama 667:1) and preparing a Sefer Torah for the next day’s reading (M.B. 667:5).