Tu BiSh'vat ("New Year of the Trees"), Jewish Arbor Day, is observed on the fifteenth (tu) of Sh'vat.
Scholars believe that Tu BiSh'vat was originally an agricultural festival, marking the emergence of spring. After the destruction of the Temple in 70 A.C.E. this holiday was a way for Jews to symbolically bind themselves to their former homeland by eating foods that could be found in Israel.
In the seventeenth century Kabbalists created a ritual for Tu BiSh'vat similar to the Passover seder. We eat an assortment of fruits and nuts and drink several varieties of wine, symbolizing different seasons or ways of being in the world.
Tu BiSh'vat has also become a tree planting festival in Israel, where both Israelis and Jews around the world plant trees in honor or in memory of a loved one.