The last day of the 7 day holiday of Sukkoth is Hoshana Rabba. The eighth day that we observe as a Yom Tov is a separate holiday named for the eighth day, Shmini Atzeres. But before we get to that, we have Hoshana Rabba, a day that is distinctive among the other days of chol hamoed -- the intermediate days of the holiday on which we are allowed to drive, spend money, etc, though we try to abstain from doing laundry and other heavy kind of work.
On Hoshana Rabba, like on the other days of Sukkoth, except the Sabbath, we take the arba minim -- the 4 species, that are the lulav [palm branch], ethrog [citron], hadasim [myrtle], and aravos [willow branches]. However, on Hoshana Rabba, the arava rises to greater prominence, carried on its own in a bunch of 5 in its own unique ceremony. The question is why should the arava, the one species that has the least to offer -- with no fruit or even fragrance -- be made the star of the show?
The answer is contained from a longer piece that you can find here: http://divreichaim.blogspot.com/2010/09/aravah-no-scent-no-taste-but-so-very.html?spref=bl
Esrog, lulav, and hadasim, representative of those with good deeds and/or Torah, are about what we have achieved. The Sefas Emes writes that these minim represent Avraham, Yitzchak, and Ya’akov. The aravah represents David HaMelech, who said of himself, “Ani tefilah,” I am prayer. The aravah’s shape resembles the lips, the conduit for our words of tefilah. Prayer is not about what we have done or can do, but about what we need Hashem to help us achieve.
When one cannot call upon the merit of good deeds, when one lacks the ability to study Torah and cannot call on the merit of scholarship, there still remains, “tefilah l’ani ki ya’atof,” the simple prayer which the poor man can wrap himself in to appear before G-d. Says the Sefas Emes, the name aravah is the same as the world arov, pleasant, because there is nothing more pleasant to G-d than listening to the prayers of those who have nothing else to offer. For this reason on the climax of the days of judgment, Hoshana Rabbah, we focus our attention not on the lulav, the esrog, or the hadasim, but rather on the aravah.
This is why the aravah belongs. No matter how many milestones we hit, the aravah reminds us that there is always more to strive for, more that we need Hashem to help us to achieve. No matter how strong our sukkos mamash, we should never forget the ananei hakavod without which our little huts would be blown away to nothing.
Note: it is customary to eat kreplach on Hoshana Rabba. You'll find the recipe here. One more thing to do on Hoshana Rabba is prepare the eruv tavshilin in anticipation of the Sabbath that follows the Shmini Atzeres Yom Tov that falls out on Thursday and Friday.