Today was the final day of the Jewish holiday of Sukkot, named for the hut - Hebrew סוכה sukkah, plural סוכות sukkot - that families traditionally put up for the duration of the holiday in commemoration of the temporary shelters used by the Israelites during their exodus from Egypt and later on during the seven-day pilgrimage of the holiday itself. However, Sukkot also incorporates aspects of a harvest festival, and one of its characteristic elements is the use of what is known as the ארבעת המינים arba'at ha-minim or "four species," namely a fruit of the אתרוג etrog or citron (Citrus medica), a green, closed frond of the date palm (Phoenix dactylifera), a leafy bough of myrtle (Myrtus communis), and a leafy willow branch (Salix sp.). This is based on the following lines in the Bible's Book of Leviticus, unfortunately often known primarily for its draconian injunctions regarding human sexual behavior and cited all too often in the name of bigotry:
On the first day you shall take the product of hadar trees, branches of palm trees, boughs of leafy trees, and willows of the brook, and you shall rejoice before the LORD your God seven days. (Leviticus 23:40)
The vague "product of hadar trees" of this translation is more traditionally read as "the fruit of beautiful trees" and taken to mean the etrog, which itself has quite an interesting history and set of associated traditions.
Festtag (Rabbiner mit Zitrone) by Marc Chagall, 1914
Interestingly, the Book of Nehemia later on in the Bible has a slightly different interpretation of the "four species":
They found written in the Teaching that the LORD had commanded Moses that the Israelites must dwell in booths during the festival of the seventh month, and that they must announce and proclaim throughout all their towns and Jerusalem as follows, "Go out to the mountains and bring leafy branches of olive trees, pine trees, myrtles, palms and [other] leafy trees to make booths, as it is written." (Nehemia 8:14-15)
Some of those identifications are again uncertain; in any case, this later version of the list - a variant reading of the passage in Leviticus? - somehow did not become entrenched as tradition.
Berlin, Adele, and Marc Brettler, ed. The Jewish Study Bible. Oxford: Oxford UP, 2005.