The 15th day of the month of Shevat marks the beginning of the “new year” for trees. Tu b’Shevat is the new year for the purpose of calculating the age of trees for tithing. The Torah states that fruit from trees which were grown in the land of Israel may not be eaten during the first three years; the fourth year’s fruit is for G-d, and after that, the fruit can be eaten. Each tree is considered to have aged one year as of Tu B’Shvat, no matter when in the year it was planted.
It is customary to plant trees and partake of the fruits of the land of Israel to mark the occasion.