Tu Bishvat 101
Tu Bishvat is a holiday intimately connected to the agricultural cycle of the Land of Israel. Falling in the middle of the Jewish month of Shvat, the 15th day of the month is the New Year of Trees. Today, this holiday is often celebrated by planting saplings and also by participating in a seder-meal that echoes the Passover seder, in which the produce of trees, including fruits and nuts, are eaten.
The Bible expresses a great reverence for fruit trees as symbols of God's bounty and beneficence. Special laws were formulated to protect fruit trees in times of war and ensure that the produce of trees would not be picked until the trees were mature enough and tithes were given from them. In order to calculate the age of trees, both for determining when they could be harvested and when they were to be tithed for the Temple, the Talmudic Rabbis established the 15th day (Tu) of the month of Shvat as the official "birthday" of trees.
Subsequent to the destruction of the Temple, Tu Bishvat lost much of its relevance, but in the middle ages it was rediscovered by Jewish mystics. In the modern period it has enjoyed another revival as a holiday that links Jews with the land of Israel and as a Jewish celebration of the environment.
When the Temple stood in Jerusalem, Tu Bishvat served as the day on which farmers offered the first fruits of the trees they planted, after the trees had turned four years old. The following Tu Bishvat signified when the farmers were allowed to begin making use of the produce of the trees they planted, whether for personal or economic reasons.