Tu B'Shevat, the 15th of Shevat on the Jewish calendar, is
the day that marks the beginning of a "New Year for Trees." This is the
season in which the earliest-blooming trees in the Land of
Israel emerge from their winter sleep and begin a new
fruit-bearing cycle.
Legally, the "New Year for Trees" relates to the various tithes that
must be separated from produce grown in the Holy Land. These tithes
differ from year to year in the seven-year
Shemittah cycle; the point at which a budding fruit is considered
to belong to the next year of the cycle is the 15th of Shevat.
We mark the day of Tu B'Shevat by eating fruit, particularly
from the kinds that are singled out by the
Torah in its praise of the bounty of the Holy Land: grapes, figs,
pomegranates, olives and dates. On this day we remember that "Man is a
tree of the field" (Deuteronomy 20:19) and reflect on the lessons we can
derive from our botanical analogue.
Taken from Chabad.org