Blessings on Customs and Hoshannah Rabbah
The question is raised a number of times by the Tosafot as to whether one recites a blessing on an act which is a observed out of custom, and not a legal obligation (see Tosafot Sukkah 44b s.v. kon be-mikdash kon be-gevulin; Taanit 28b s.v. amar shema mina minhag avoteihem be-yedeihem; Berachot 14a s.v. yamim she-ha-yahid gomer bahem et ha-hallel). In relation to Hoshannah Rabbah, the Talmud asks as to what is the origin of taking willow branches on Hoshannah Rabbah.
It was stated, R. Johanan and R. Joshua b. Levi differ. One holds that the rite of the willow-branch is an institution of the prophets (יסוד נביאים), the other holds that the willow-branch is a usage of the prophets (מנהג נביאים). It can be concluded that it was R. Johanan who said, ‘It is an institution of the prophets’, since R. Abbahu stated in the name of R. Johanan, ‘The rite of the willow-branch is an institution of the prophets’. This is conclusive. (Sukkah 44a)
The importance of the distinction between יסוד נביאים and מנהג נביאים is that the former requires a blessing, while the latter does not. The Talmud continues,
Aibu related, I was once standing in the presence of R. Eleazar b. Zadok when a man brought a willow-branch before him, and he took it and shook it over and over again without reciting any benediction, for he was of the opinion that it was merely a usage of the prophets. Aibu and Hezekiah, the maternal grandsons of Rab, brought a willow-branch before Rab, and he shook it over and over again without reciting a benediction, for he was of the opinion that it was merely a usage of the prophets. (Sukkah 44b)
The Tosafot discuss a number of instances where actions that have the status of custom (מנהג) are performed with a blessing, e.g. Hallel on Rosh Hodesh (at least for Ashkenazim) and the second day of Yom Tov. Rabbeinu Tam’s conclusion is that a distinction must be drawn between those instances when a blessing is recited, and the taking of the willow on Hoshannah Rabbah which is “just taking” it, “טלטול בעלמא” (Tosafot Taanit, ibid.). While most poskim followed the opinion of Rabbi Joshua b. Levi that the taking of the willow on Hoshannah Rabbah is a custom of the prophets, it is reported that Rav Shmuel ben Hofni Gaon agreed with the opinion of Rabbi Yohanan that it was “an institution of the prophets”, hence requiring a blessing (see Rosh on Sukkah, 4:1).
Moadim le-Simha.