Tu Bishvat is the new year for trees. According to the Mishnah, it marks the day from which fruit tithes are counted each year, and marks the timepoint from which the Biblical prohibition on eating the first three years of fruit and the requirement to bring the fourth year fruit to the Temple in Jerusalem were counted. In modern times, it is celebrated by eating various fruits and nuts associated with the Land of Israel. During the 1600s, Rabbi Yitzchak Luria of Safed and his disciples created a short seder, called Hemdat ha‑Yamim, reminiscent of the seder that Jews observe on Passover, that explores the holiday's Kabbalistic themes.
(Adapted from Wikipedia)