Mitzvah HaBa'ah B'Aveirah on Yom Tov Sheini: Zecher L'Mikdash versus Mitzvah D'Rabbanan

The shiur opens with the Gemara (גמרא)'s discussion of why a yaveish (dried) lulav or a gazul (stolen) lulav is permitted on Yom Tov Sheini but not on Yom Tov Rishon. Rashi (רש"י) explains that Yom Tov Rishon is d'oraysa, requiring hidur mitzvah (מצוה), while Yom Tov Sheini is d'rabbanan and does not. Tosfos asks: if the din of hadar (beauty) applies all seven days, why doesn't chaser (missing component) also apply all seven days? Tosfos distinguishes between what comes before "u'lekachtem lachem" in the pasuk (which applies only on day one) and what comes after (which applies all seven days). The Ramban (רמב"ן) reads the entire sugya as dealing with d'oraysa categories, arguing that chaser is a pesul all seven days in the Mikdash according to the din of hadar, not lekichah. Rabbi Zweig then introduces a fundamental question about the nature of the rabbinic enactment of taking lulav seven days in the provinces after the Beis Hamikdash was destroyed. The Gemara establishes this practice as "zecher l'mikdash" based on the pasuk "Tzion hi doresh ein lah." The opening assumption of the Gemara is that there is no reason why yaveish or gazul should be pasul on days 2-7—they should be kosher because the practice is merely a zecher, a commemoration, not a full mitzvah. This is the exact opposite of what one would expect if it were a mitzvah d'rabbanan structured like the d'oraysa mitzvah.