The Mishna describes how the mitzva of aravah was done.
There was a place called Motza, which is a village just a few kilometers to the south of Jerusalem, where the aravot were gathered for use in the Temple. This village still exists; it is first mentioned in Sefer Yehoshua (18:26-28) as one of the cities of the tribe of Binyamin. In the time of the Mishna the Romans established it as a garrison town to house the soldiers who protected Jerusalem. Apparently this was a place with unique willow trees whose branches were long enough to lean over the altar when they were placed next to it.
Every day of Sukkot, the people would circle the mizbe’ah (altar) one time, and on the seventh day they would walk around it seven times. The Talmud Yerushalmi explains that this was done in remembrance of the victory in Yeriho (see Yehoshua chapter 6), when the Jewish people circled the city once a day for six days and seven times on the seventh day before the walls of the city collapsed. The Arukh LaNer comments that this fits in with the theme of the holiday of Sukkot, which celebrates specifically God’s miracles on behalf of the Jewish people in the land of Israel. In a similar vein, the Maharsha says that on Sukkot we are obligated to commemorate the miracles that God did on our behalf, which is why we invoke a memory of the public miracle of the walls of Yeriho collapsing.
Upon completing the procession around the mizbe’ah, the people would say yofi lekha mizbe’ah, yofi lekha mizbe’ah – proclaiming the beauty of God’s altar. The Arukh LaNer explains that there was a particular reason to compliment the altar on Sukkot, either because it was the focus of the processions that take place on the holiday or because more sacrifices are brought on Sukkot than on any other holiday.