Sukkah, Chapter 4, Mishnah 6. The previous Mishnah dealt with the mitzvah of the aravah, and our Mishnah continues that very same discussion.
"Kamah she'asahu bachol, kach asahu b'Shabbos" - the mitzvah of the aravah is performed on Shabbos exactly as it is performed on weekdays, with no change.
The one difference between weekday and Shabbos:
The Mishnah teaches that they would gather the aravos on Erev Shabbos - cutting the willow branches before Shabbos came in - and place them in golden basins, vessels made of gold, so that they would not wilt: standing in water, they retain their moisture and do not wither.
The opinion of Rabbi Yehudah ben Beseira:
Rabbi Yehudah ben Beseira offers a far reaching suggestion: instead of aravos they would bring young palm branches, that is, branches of the date palm - palm fronds - and beat them on the ground at the sides of the Mizbe'ach. This was the act performed in place of the aravos.
That day was called "Yom Chibut Chariyos" (the day of beating the palm branches). This does not refer to one specific, defined day, but to those days of Sukkos on which they would bring the palm fronds instead of the aravos, and because of the beating of the branches the day was given this name.
The discussion of the Rishonim regarding this opinion:
Some understand, based on the Gemara, that Rabbi Yehudah ben Beseira agreed that the mitzvah of aravah was indeed practiced, and that taking the palm branches was an addition to it.
And some understand that the practice of beating the palm branches was unique to Hoshana Rabbah alone - on that day they would take young palm branches, but not on the other days of the festival.
In summary: In this Mishnah we learned that the procedure of the aravah on Shabbos is identical to its procedure during the week, except for gathering them on Erev Shabbos and placing the branches in golden basins so that they would not wilt; and the opinion of Rabbi Yehudah ben Beseira regarding young palm branches beaten at the sides of the Mizbe'ach, after which the day is called "Yom Chibut Chariyos"; and the dispute among the Rishonim as to whether this was an addition to the aravah or a practice unique to Hoshana Rabbah alone.