Related for Arakhin 19:2

The [beat of the willow] is a tradition from Moses at Sinai, and Abba Sha'ul deduced it from Scripture, as it is says, “Willows of the brook”, the plural denoting two, one for the lulav, and one for the altar. Rabbi Elieser ben Yacov said, Thus were they saying, "To Him and to thee, O altar, to Him and to thee, O altar!" Eighteen days and one night (in the year) the entire Hallel is repeated. These are: the eight days of sukkot, the eight days of Hanukkah, the first day of Passover, the night of the first day of Passover, and the first day of Shavuot.

And the Levites with their harps and lyres and cymbals and all manner of musical instruments without number were there, saying, “Behold, bless ye the Lord, all ye servants of the Lord.” Some were saying, Lift up your hands to the sanctuary, and bless ye the Lord. When they parted they said to one another, The Lord bless thee out of Zion, and see thou the good of Jerusalem all the days of thy life. You should see your children's children. The herald cried out: they sounded a plain note, a tremolo, and a plain note. Rabbi Yehudah said: They did not sound less than seven nor more than thirteen times at the opening of the Temple gates. He who blew at their opening did not do so at their closing. Three times they sounded before the altar. He who blew before the altar did not do so on the tenth step, and he who blew on the tenth step did not do so before the altar.