Overview
Lulav is a closed frond of the date palm tree. It is one of the Four Species used during the Jewish holiday of Sukkot. The other Species are the hadass (myrtle), aravah (willow), and etrog (citron). When bound together, the lulav, hadass, and aravah are commonly referred to as "the lulav".
Codification in the Torah
The Torah mentions the commandments to obtain a lulav for the Sukkot holiday once in Leviticus:
Leviticus 23:40
"And ye shall take you on the first day the fruit of goodly trees, branches of palm-trees, and boughs of thick trees, and willows of the brook, and ye shall rejoice before the LORD your God seven days."
In the Oral Torah, the Mishnah comments that the biblical commandment to take the lulav, along with the other three species, is for all seven days of Sukkot only in and around the Temple Mount when the Holy Temple in Jerusalem is extant, as indicated by the verse as "in the presence of Hashem, your God, for seven days." In the rest of the Land of Israel, as well as in the Diaspora, the four species are biblically mandated only on the first day of Sukkot. After the destruction of the Temple in 70 CE, Rabbi Yochanan ben Zakai legislated a rabbinical enactment to take the four species for the entire seven days of the holiday in all locations as a commemoration of what was done in the Temple.
From Wikipedia (CC BY-SA 4.0).