The minor holiday of Tu B'Shevat (the 15th of Shavat) is not mentioned in the Bible. It is first brought up in the Talmud, as the "New Year of trees." This refers to tithes collected from fruit bearing trees. Tu B'Shevat is to tithing what Dec. 31st is to our fiscal year - the time to close out the books.
That sounds bluntly administrative, but there's more. In terms of the natural order, in Israel this date roughly marks the winding down of the harshest pounding winter rains (the name for the month, Shevat, means "club"). The rains are a pain, but they are also vital to the vitality of the coming growing season (Israel depends almost entirely on rainfall) so this signals the first hints of renewed life. And trees are emblematic of this hope.
Tu B'Shevat also has a metaphysical importance because of the rain and the trees. Trees, in particular, are one of the most potent symbols of and for Judaism. Rooted in the ground and reaching for the heavens, it is a symbol of Torah, the human being, and the Sefirot (the divine order).
In various traditions, we learn that trees are sentient and offer praise to God continuously (Gen., Rabbah 13:2; Perek Shirah). The cosmic trees in the center of Eden, the Tree of Knowledge of Good and Evil (all things, which stands for the universal order we we know, the unredeemed world) and the Tree of Life (understood by Judaism to be the Torah and the source of immortality and the ideal divine order God wants us to restore to creation) are not the only trees of power. All trees in Eden have the power to heal and give off a scent that comforts a soothes the soul - symbolic of the centrality of trees to human well-being even to this day. At the conclusion of most services, quoting the Proverbs, we praise the Torah for it tree-like capacity to nurture, sustain, and protect us ("...It is a Tree of Life for all who cling to it...").
Not surprisingly, then, we find that the mystics of Safed developed a mystical seder in honor of the holiday. This lovely service, involving wine and the eating of various fruits, urges us at a time of torpor and dreariness toward re-sensitizing us to the essential rhythms of the earth and the pulse of divine stirrings.