Tu B’Shavat is the Jewish new year for the trees and is celebrated in the winter on the 15th of Shevat. Leviticus 19:23-25 says that one cannot eat the fruit from a tree for the first three years, and in the fourth year it is a praise offering unto the L-rd. After the fourth year it can be eaten. Each tree is considered to be aged one year as of Tu B’Shevat, which literally translates, the 15th of Shevat.
The most common tradition is to plant trees or raise money to plant trees in Israel. Some Jews eat fresh fruit or have a special sedar with the the seven species: wheat, barley, grapes (vines), figs, pomegranates, olives and dates (honey) (Deut. 8:8).
Tu B’Shevat isn’t mentioned in the Torah and is mentioned only once in the Mishnah, so it may be considered less important than the Leviticus 23 feasts of the Lord.