1. The Gemara explains why we make a seven-day remembrance for Lulav but not for Aravah.
2. There is a Torah law of Aravah, and there is a law or custom that was instituted by the prophets.
3. The custom or law (#2 above) is that each person must hold Aravos separately on the seventh day of Sukos.
4. There is a dispute about how many Aravos one must hold.
5. One does not recite a blessing on the holding of the Aravah.
A BIT MORE
1. Since the Mitzvah of Lulav was done during the time of the Beis ha'Mikdash by people who were not in the Beis ha'Mikdash, a remembrance was established for seven days outside the Beis ha'Mikdash as well. However, the Mitzvah of Aravah was a special service, performed only in the Beis ha'Mikdash and, therefore, no remembrance was established for it after the Beis ha'Mikdash was destroyed.
2. The Torah law is that the special service of Aravos be done in the Beis ha'Mikdash. The prophets instituted a law or custom that even outside the Beis ha'Mikdash, Aravos should be held on the seventh day of Sukos.
3. One does not fulfill this custom by holding the Aravos that are tied together with the Lulav, even if he picks up the Lulav and Aravah for this purpose.
4. Some say that one must hold three Aravos, each one with wet leaves. Others say that even one Aravah with one leaf on it is sufficient.
5. One does not recite a blessing for a "Minhag Nevi'im."