Daf Yomi - Sukkah 43 - Hoshanos on Shabbos

The Gemara states that the Chachamim would always ensure that the seventh day of Sukkos, known as Hoshanah Rabbah, would not occur Shabbos. Tosfos wonders why it was more important to ensure that the seventh day of Sukkos not occur on Shabbos as opposed to ensuring that Rosh HaShanah or the first day of Sukkos do not occur on Shabbos. On both Rosh HaShanah and the first day of Sukkos there are biblical mitzvos to perform. The Chachamim should have been more concerned with ensuring the observance of these mitzvos than ensuring the continuity of the aravah mitzvah which is only a rabbinical institution to commemorate the mitzvah that was performed in the Bais HaMikdash. Tosfos answers that there was no concern that people would assume that the mitzvah of blowing shofar on Rosh HaShanah and taking the lulav on the fist day of Sukkos were rabbinical in nature. Regarding the mitzvah of aravah, however, if the seventh day of Sukkos would occur on Shabbos and the aravah would not be taken, people would view the custom as irrelevant and they would not take the aravah in the future. Furthermore, if the seventh day of Sukkos occurs on Shabbos, there would not be another opportunity to take the aravah, because one would not be able to take the aravah on Shemini Atzeres because it may be an independent festival and taking the aravah would contradict the theme of the day. Regarding the mitzvah of shofar and lulav, however, one would be able to perform the mitzvah on the second day of the festival.