The 15th of Shevat (that's today, Saturday) is the "New Year" for trees (there are four new years in Judaism), and is marked by eating of the seven kinds, sometimes at in a seder setting. These are the fruits and grains that Torah relates with the Holy Land's fertility: wheat, barley, grapes, figs, pomegranates, olives and dates (Deut. 8:8). There is, to some such as the Kabbalists, a significance to each kind.
Many relate to the significance of trees by means of establishing and caring for roots -- that which connects us to our past and promises us a fruitful future. Torah states: "Man is like the tree of the field" (Deut. 20:19). The Lubavitcher Rebbe suggested that
We are trees, living two lives at once. One life breaking through the soil into this world. Where, with all our might, we struggle to rise above it, grapple for its sun and its dew, desperate not to be torn away by the fury of its storms or consumed by its fires.
Then there are our roots, deep under the ground, unmoving and serene. They are our ancient mothers and fathers, Abraham, Sarah, Isaac, Rivkah, Yaacov, Leah and Rachel. They lie deep within us, at our very core.
For those interested, the legality of the day relates to tithes and what must be set aside according to Torah.
When you come to the land and you plant any tree, you shall treat its fruit as forbidden; for three years it will be forbidden and not eaten. In the fourth year, all of its fruit shall be sanctified to praise the Lord. In the fifth year, you may eat its fruit. (Lev. 19:23-25)The celebration of this activity is not mentioned specifically in Torah, but is apparent in Mishnah. Aside from seders and fruit consumption, the day is also sometimes viewed as the Jewish "Arbor Day." If I had a lawn, perhaps I'd plant a tree. But alas. Instead, I'll find me some grapes and olives and nosh.