One of the topics discussed in our perek is how one can prepare for Shabbat when Yom Tov occurs on Friday. Rabbah suggests that we rely on a legal fiction called ho’il – since guests might come to visit on Yom Tov, one can prepare food on Yom Tov (which is permissible when done for that day) with these theoretical guests in mind. When it turns out that there is food left over for Shabbat, Shabbat is thus prepared for. Rav Hisda argues that on a Biblical level one can prepare for Shabbat on the Friday on which Yom Tov occurs; the prohibition is a Rabbinic one, lest someone prepare for a regular weekday on Yom Tov. The Sages instituted an eruv tavshilin, which clarifies that the preparations can be done only for Shabbat.
Rabbah challenges Rav Hisda’s ruling from the law regarding the lehem ha-panim – the showbread of the Temple.
The laws of the Temple showbread appear in Vayikra 24:1-9 and are discussed at length in the Talmud in Massekhet Menahot. Twelve loaves were baked every week, which were placed on the shulhan in the heikhal on Shabbat. They remained there until the following Shabbat, when they were replaced by freshly baked loaves. The loaves were then distributed among the groups of kohanim who were working in the Temple.
The Mishna in Menahot (11:9) teaches that ordinarily the lehem ha-panim was eaten nine days after it was baked (baked on Friday and eaten the following Shabbat). When Yom Tov fell on Friday, it was eaten ten days after it was baked; when Rosh ha-Shanah fell on Thursday and Friday, it was eaten eleven days after it was baked. Clearly the Mishna believes that the baking cannot be done on Yom Tov in preparation for Shabbat, seemingly against Rav Hisda’s ruling.
Rav Hisda responds by pointing out that, in this case, the preparation is not for the immediately upcoming Shabbat, as the bread was not eaten for more than a week! Preparation for a week later would be permitted on Yom Tov. When Shabbat immediately follows Yom Tov, however, food preparation for Shabbat would be permitted.