The Mishna tells us that a dry lulav and a stolen lulav are both pasul. The gemara (Sukkah 29b) deduces from the fact that both cases are lumped together that they are similar. Therefore, just as a dry lulav is pasul for all 7 days of the chag, so too, a stolen lulav must also be pasul for all 7 days. The sugya then goes on to explain the source for that din.
Rashi explains why a dry lulav is pasul for all 7 days:
בשלמא יבש. פסול בדרבנן נמי כיון דמצוה הוא משום זכר למקדש בעינן הדור מצוה
Why does Rashi need to mention in this context that the taking of lulav all seven days is a zecher l'mikdash? Why is that relevant? Wouldn't it have been enough to just tell is that there is a psul of hadar that applies to the chiyuv derabbanan of taking lulav for seven days?
I haven't found anyone who explains this and I am stumped. Maybe you have a hesber?
Had you asked me, I would have said that we should say hoshanos on Pesach. That's when the farmer has just planted his crops, nothing is growing yet, and so that's the time you want to cry out, "Ana Hashem hoshi'a na, Ana Hashem hatzlicha na." That's when you want to do nisuch hamayim = shifchi ka'mayim libeich. Sukkos is chag ha'asif, the harvest season, when the storehouse is full of grain, when you reap the fruits of a years worth of labor. Why say hoshanos now?
There is no kuntz to davening and having bitachon when you are facing need and crisis. It's the guy who needs parnasa, who needs refu'ah, who needs help for his children and family, who is davening a 10 minute shmoneh esrei in shul. The guy who mumbles the words in 3 minutes so he can race out the door to drive to a well paying job in a fancy car and come home to a house in order with everyone in good health and happy is davening a different davening (of course, if he took a moment he might realize that even if he has no needs, he could put in a good word for his friend). What Sukkos comes to teach us is that even that second guy should be davening that 10 minute shmoneh esrei. Even when times are good, you have to thank Hashem for what you have and recognize that its all from him and that you are completely dependent upon him. That's why davka now, when the storehouse is full, when the harvest is finished, we say hoshanos, we pour our hearts out in tefilah. Bitachon and dveikus is not just for when we are lacking, but its even for when we have it all. "V'ha'boteiach ba'Hashem chessed y'sovivenu. (Tehillim 32:69) Bitachon, says the Sefas Emes (5645), is the midah of sukkos, which surrounds us and envelops us within.
Chazal argue whether sukkah is a diras keva or a diras arayei, a permanent structure or a temporary dwelling, and we pasken like the latter, majority view. It takes a lot of bitachon to put aside your home, your life, and go live in a tent like a nomad, with nothing. That's the level of bitachon sukkos is all about. For big tzadikim, this is a diras keva, this is the way they live all year. For me, for most of us probably, this is a diras arayei, a temporary moment. We all know that after a week we will be back inside, back to work, back to the same grind. We can't meditate on this idea of bitachon and live with it for awhile, but we will come back to earth, right? After walking away from his encounter with Eisav unharmed, Ya'akov builds for himself a bayis -- that new level is where he would be at from now on. But for his possessions and his flocks, he made sukkos, little temporary huts. The Torah tells us that he named the place he was at not bayis, but Sukkot -- that was what he thought was most important. The big tzadikim of course are on a high level all year -- that's what we expect. What Ya'akov thought was more significant, more of an accomplishment, is that everything else, everyone else, can at least step into that mode for a short while, for a temporary visit. "Yafeh sha'ah achas b'teshuva u'ma'asim tovim b'olam ha'zeh." Even if it's only for a week, that week can still make a difference.
V'lakachtem lachem -- you have to take yourself. The gemara darshens on "v'lakachtem lachem" that "md'agbihei nafik bei," that you just need to lift up the lulav and you are yotzei. We need to take ourselves and lift ourselves up. Even for one week, even temporarily. The effect can last a whole year.