The 15^th day of Shvat, Tu B'Shvat, is called the New Year for Trees. Strictly speaking, this title draws a legal distinction related to the laws of tithing in the Land of Israel. Tithes must be separated from any produce grown in Israel before it may be eaten. In a given year, the fruit taken as tithe from one tree may represent the owner's other trees of the same species. However one year's fruit may not be tithe for another year's harvest.

Tradition teaches the new year for fruit begins on the fifteenth of Shevat, because most of the winter rains will have passed and the sap of the new growth has begun to flow: the dormant tree is waking from its winter sleep. A tree that blossoms before Tu B'Shvat is considered last year's produce; if it blossoms after Tu B'Shvat, it belongs to the new year.

Other than the day's significance for tithing, there is no source in the Talmud or Midrash for celebrating Tu B'Shvat. Yet from later sources we find many customs regarding the celebration of Tu B'Shvat: the practice of eating various fruits; the custom of dressing in one's Shabbos finery for the new year for trees, because the Torah compares the human being to a tree (Deuteronomy 21:19).

Excerpted from the article "A View from the Trees" By Rabbi Ephraim Nissenbaum, Aish.com