Tu BiShvat, the fifteenth of Shevat, began as a strictly administrative marker rather than a festival in any devotional sense. The Mishnah, in its opening tractate, names the date as the new year for trees, meaning the cutoff by which a fruit was counted toward one tithing year or the next, a technical boundary for the biblical tithing laws governing fruit trees rather than an occasion carrying ritual observance of its own.
The date acquired a devotional character many centuries later, through the circle of Kabbalists gathered in the town of Safed in the sixteenth century, working in the tradition of the mystic Isaac Luria, whose school later produced the Tu BiShvat seder known as the Pri Etz Hadar, first published in 1728, built around the symbolic eating of different categories of fruit read as corresponding to layers of the mystical cosmos, modeled loosely on the Passover seder. In the twentieth century the date acquired a third layer of meaning again, taken up by the modern Zionist movement and organizations such as the Jewish National Fund as an occasion for tree planting in the Land of Israel, so a date fixed originally for tax purposes now carries an administrative origin, a mystical ritual and a modern environmental association layered one on top of the last.