Tu B'Shevat, the 15th day of the Jewish month of Shevat (normally around January – February), is a holiday also known as the "New Year for Trees". The Hebrew word "Tu" is actually the number 15, not a word.
Rabbinic Judaism, like many cultures, has several different "new years." We in “the West”, for instance, have the calendar year (January - December), the academic year (September - June), businesses have company fiscal years, and the fiscal tax year is April - March. It's the same principle with the various Biblical and Jewish “new years”.
Tu B'Shevat is the new year for calculating the age of trees for tithing (see Leviticus 19:23-25). It states that fruit from trees may not be eaten during the first three years; that the fourth year's fruit is for God and, from then on, you can eat the fruit yourself. Each tree ages one year on each Tu B'Shevat.
As such, if you planted a tree on Shevat 14, it would start its second year the very next day, whereas a tree planted two days later, on Shevat 16 would wait nearly a full year before it "turned 1 year old".
There are not many customs or observances related to Tu B’Shevat. One is to eat fruit, or even to eat from the Seven Species (shivat ha minim). The shivat ha minim are described in the Bible as being “abundant in the land” and include: wheat, barley, grapes, figs, pomegranates, olives and dates (see Deuteronomy 8:8).
One of the loveliest traditions is to plant a new tree on this day. Several charities exist in Israel to help re-populate the Land with trees and make it a flourishing landscape.