The 15th of Shvat is the "New Year of the Trees" (Tu BiShvat in Hebrew). The date itself is related to the rainy season, which begins in the Land of Israel with the festival of Sukkot. It takes four months of rain to saturate the soil, nourish the trees, and make them bear fruit. This is important to know in order to separate tithes from the fruit, as was done in the time of the Temple, since the tithes varied from year to year.
Human beings also "celebrate" alongside trees. The Torah says, "Man is a tree of the field." We are nourished by deep roots, reaching back to Abraham and Sarah; we reach up to the heavens while standing firmly on the earth; and when we act accordingly, we produce fruits that benefit the world.
It is customary to eat fruit on that day, particularly those with which the Land of Israel is praised: olives, dates, grapes, figs, and pomegranates.