We have already established that outside of the Temple, on a biblical level the mitzva of lulav is only on the first day of the holiday; our tradition of taking the lulav and etrog for the entire seven days of Sukkot is zekher le-Mikdash – a commemoration of the Temple where it was a mitzva to take the lulav every day of the holiday (see Sukka 41).
This is summed up in our Gemara, where Rabbi Yehoshua ben Levi declares that only the first day is the mitzvat lulav (the commandment of lulav); the rest of the week is mitzvat zekenim (the commandment of the Elders).
The Rashash argues that there is a practical difference being suggested here. According to Rabbi Yehoshua ben Levi, only on the first day should a person bless asher kidishanu bemitzvotav vetzivanu al netilat lulav – that we are commanded in the mitzva of taking a lulav. On other days the blessing that should be recited is asher kidishanu bemitzvotav vetzivanu al mitzvat zekenim – that we are commanded in the mitzva of following the words of the Elders. This is also indicated in the Talmud Yerushalmi.
The Gemara notes: And Rav also held that the blessing over the mitzva of lulav is recited all seven days, and one recites the blessing even on the six days when the mitzva is rabbinic law, as Rabbi Hiyya bar Ashi said that Rav said: One who lights a Hanukkah light must recite a blessing.
According to Rav, the mitzva of lulav is for the entire week, even if its basis is a Rabbinic enactment. This is apparent from Rav’s ruling with regard to Hanukkah candles, where the blessing that is recited is asher kidishanu bemitzvotav vetzivanu lehadlik ner shel Hanukkah – that we are commanded in the mitzva of lighting the Hanukkah candles.
In answer to the Gemara’s query “where are we commanded in this mitzva?” (after all, the Hanukkah story takes place during the Second Temple period, well after the Torah was written), the pasuk of lo tasur (see Devarim 17:9-11) is given, which indicates that we must listen to the words of the priests and judges of our time.