1. The Mitzvah:

On Hoshana Rabbah, an Arava is taken in addition to the Aravos that are bound with the Lulav.[1] It is taken in commemoration of the Hakafos that were performed with the Aravos in the Temple, in which the Mizbeiach was encircled with the Arava branch.[2] These Aravos are also referred to as Hoshanos.[3]

Blessing: A blessing is not recited upon fulfilling the Mitzvah of Aravos for Hoshanos.[4]

[1] Michbaer 664:2; Sukkah 44b

[2] Taz 664:2

[3] See Michaber 664:8 and Rama 664:9

[4] Michaber 664:2