Related zu Wajikra 23:40

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.