Halakhah do Wyjścia 29:36

And there is also another matter with these four things - that they are similar to precious limbs in a man (see Vayikra Rabbah 30:14; Midrash Tanchuma, Emor 19): As the citron is similar to the heart, which is the dwelling place of the intellect, to hint that he should serve His creator with his intellect; the lulav is similar to the backbone, which is the essence of a person, to hint that he should straighten himself completely for His service, blessed be He; the myrtle [leaves are] similar to the eyes, to hint that he should not stray after his eyes 'on the day of the rejoicing of his heart'; and the willow [leaves are] similar to the lips, with which a man completes all of his acts of speech, to hint that he should put a muzzle to his mouth, calibrate his words and fear God, may He be blessed, even at a time of joy. And the reason that it is only practiced one day in the country (outside of the Temple) is well-known - since the main joy is on the first day. And if you should ask, "Why would one not take it on Shemini Atseret, which has great joy on it for Israel" - the answer is that the day of Shemini Atseret is completely for God, may He be blessed. And [it is] as they, may their memory be blessed, said (Bemidbar Rabbah 21:22; Midrash HaGadol, Shemot 29:36), "A parable of a king who made a feast, etc.," as it is [found] in the Midrash. And at the end, He said to them, "Remain with me one day, as your departure is difficult for me." And therefore, it is called atseret (a stopping). And if so, there is no need for any other reminder. And the holiday of Pesach does not require another reminder with the lulav, as behold the matsa and marror and the body of the Pesach sacrifice are between his hands; and further since it is not a time of joy as [much as is] the holiday of the gathering. And the holiday of Shavuot also does not require another reminder, since the essence of [that] festival is only from the angle of the giving of our Torah - and that is the great reminder to straighten our ways. And this is what appears to me in these matters from the side of the simple understanding. And I have come to believe that the kabbalists (mystics) have wonderful secrets about the commandment of the lulav and the three [other plants].