Tu Bishvat is the new-year celebration ("Rosh Ha'Shana") for the trees. It is not as well known as some of the other holidays, and is celebrated mainly (but not only...) in Israel.
The origin of this holiday is not in the bible, but the time of the Mishnah (around 2000 years ago). At that time, fruit and vegetables were paid to the landlords as a form of taxation. In the 16th century, the Kabbalists in the city of Tsfat turned this occasion into a festival by accompanying their prayers on Tu Bishvat with wine and fruit. This became a Jewish custom around the world, in memory of the holy land and its fruits. This was also the origin of a custom which developed later, the Tu Bishvat Seder.
At the end of the 19th century, when Jews began settling in Israel once again, Tu Bishvat received new significance and became not only the new year day, but also the celebration of planting trees. Since the foundation of Israel, children are taken by their schools every year to plant trees, and so the holiday emphasizes the importance of looking after nature and the environment. Tu Bishvat also praises the Holy Land and God - for giving us Israel and its fruits, specifically the Seven Species.
Tu Bishvat Seder
A feast including drinking both red and white wine, and eating Israeli fruits (such as figs, dates, nuts, etc.). The participants of this event read quotations from the bible in which the typical Israeli plants are mentioned, tell the history of the holiday, and sing songs about nature and trees in Israel.
The Seven Species
Of all kinds of trees, bushes, fruits and vegetables in Israel, seven species are mentioned in the bible as those with which Israel is blessed: wheat, barley, grapes, pomegranates, dates, figs and olives.