"You shall take for yourselves on the first day a fruit of a citron
tree, branches of palms, twigs of myrtles, and brook willows."
For the seven days of Sukkos, excluding Shabbos, we are commanded to take
a lulav, an esrog, three myrtle branches, and two willow
branches, which we lift up and shake each day. These four species
symbolically correspond to a person's spine, heart, eyes, and lips, to
show that we dedicate each part of ourselves to serving Hashem (Vayikra
Rabbah 30:13), as the verse says, "All my parts say, 'Hashem, who is
like You?'" (Tehillim 35:10)
The Rambam (Moreh Nevuchim 3:43) explains that the four species
serve as a reminder to rejoice in the fact that Hashem took us from the
wilderness to a country full of fruit trees and rivers. These four
species have a pleasant appearance and cause us to be joyous.