The holiday of Sukkot falls out after Yom Kippur for good reason. After reaching levels of angelic purity we return to a holiday of physicality indicating that out true mortality exists in the physical and that this is the preferred way of serving G-d.
On Sukkot one must attain the four species, the Etorg, Lulav (palm branch), Hadassim (myrtles branches) and Aravot (willow branches) and hold them together.
By examining the two explanations of what the Etrog, Lulav, Hadasim and Aravot come to represent we can understand why they must be held together and achieve greater clarity into the significance of the Etrog.
One explanation is that the Etrog which smells and tastes good represents the righteous, the Lulav which has a good taste but no smell represents a person with knowledge but with no good deeds, the Hadas which has a good taste but no smell is like a person with no knowledge but good character traits and the Arava has no taste and no smell symbolic of those with no knowledge and poor character traits.
So why are such diverse elements held together in the hand? The answer is that the Divine is seeking His whole nation, whatever their stature to be bound together and serve Him as one. The righteous, symbolized by the majestic Etrog, have the great task of uniting, teaching and leading those with lesser knowledge and character. No Jew must be left behind in the service of G-d and therefore this unification is symbolized through holding these four species together.
The second explanation of what the four species represent is based on their shape. The Etrog is the shape of the heart and represents the emotions of man. The Lulav resembles the spine of a man, symbolizing uprightness, the Hadas corresponds to the eyes, enlightenment, and the Arava represents the lips, symbolic of prayer. The message is that the body must be unified in thought and action to serve the Divine. Namely, one must be straight and unbending in his service of G-d but must utilize his heart to reach an emotional nirvana which can only be attained though using his eyes and mouth as spiritual vehicles.
