Leviticus 23:39-40

39‘On exactly the fifteenth day of the seventh month, 1when you have gathered in the crops of the land, you shall celebrate the feast of the Lord for seven days, with a 2rest on the first day and a 3rest on the eighth day.

40Now on the first day you shall take for yourselves the 4foliage of beautiful trees, palm branches and branches of trees with thick branches and willows of the brook, and you shall rejoice before the Lord your God for seven days.