Happy New Year. No, I am not giving a late greeting for the civil new year celebrated a few weeks ago, a day that the Romans and the Talmud called calenda but which, for some strange reason, Israelis insist upon calling by an obscure Christian name, Sylvester. Rather I am anticipating Tu Bishvat, the new year of the trees, that will be celebrated next Tuesday. Tu Bishvat, the 15th day of the month of Shvat, is hardly a major holiday. It began really only a calendar date for the counting of the number of years a tree has borne fruit for purposes of tithing and so that we know when it is permissible to use that fruit. It is not found in the Torah but is mentioned first in the Mishna where we are told that there are four "new years" - the first of Nisan for kings and holidays, the first of Elul for animal tithes, the first of Tishrei for years and shmita and the first of Shvat for trees - this is according to the School of Shammai, while the School of Hillel says that it is the 15th of the month (Rosh Hashana 1:1). It is fascinating that only this one new year was a subject of disagreement. The ruling of the School of Shammai that it was to be observed on the first of the month seems eminently sensible. Why commence a new year, a time of counting, in the middle of the month as the School of Hillel ruled? Yet that is the ruling that we follow, otherwise we would be celebrating Ehad Bishvat! One of the greatest talmudic scholars of the last century, Louis Ginsberg, once addressed that question in a lecture he gave at the Hebrew University and later published in his book On Jewish Law and Lore. The conclusion that he came to was that this decision was based upon socio-economic factors: The simplest explanation is that the rich possessed good and fruitful fields on which trees began to blossom a week or two before the blossoming of the trees on the meager and unyielding soil of the poor; therefore the date of the new year of the trees is the first of Shvat according to the School of Shammai and the 15th according to the School of Hillel. Ginsberg also cites many other areas in which the decisions of the two schools were influenced by the socio-economic classes they represented. Among others he mentions the minimum amount to be spent on sacrifices (Hagiga 1:2), the removal of shutters on shops on festival days (Beitza 1:5) and the definition of a generous gift of teruma (Terumot 4:3). Thus the first lesson we learn from Tu Bishvat is that Jewish law was frequently determined by practical matters, taking into account the financial needs of the people. It is not accidental that Jewish law follows the School of Hillel in all but a few matters. It is not only a question of leniency, but also of making it possible for the majority of people, even those of the poorer classes, to observe it. The decision of Rabbi Abraham Isaac Kook concerning shmita, then, was not without good precedent. The second lesson of Tu Bishvat is that religious practice can grow and change with the times. Over the centuries Tu Bishvat acquired new customs and significance. Since it was connected to the flowering of fruit trees, in medieval times the custom arose of eating fruit on that day and especially fruit common to the Land of Israel. For Jews living in other lands this was yet another way of connecting to the land. Later kabbalistic customs arose in which a seder was held, patterned after the Pessah Seder but stressing different kinds of fruits and wines. The latest reincarnation of Tu Bishvat, of course, occurred with the growth of Jewish agriculture here in pre-state Palestine, when the fact that it was connected to trees - the new year for trees - gave birth to the idea that this would be a day for celebrating trees by planting them and thus renewing the forests that had disappeared here. Indeed that became the dominant observance of Tu Bishvat to this day, for which the Jewish National Fund is exceedingly grateful. Today in many places other concepts have been added stressing the preservation of our green planet and ecological considerations. And so a day that originally was no more than a calendar date important for ritual purposes grew, changed and expanded creatively to emphasize the importance of the fruits of Israel, of agriculture, of renewing and reforesting a barren land and now of protecting our planet. This is a wonderful example of the way in which old forms can be renewed and invested with new and meaningful ideas. There is nothing static about Jewish law or Jewish observance. The writer is an author and lecturer who serves as the head of the Rabbinical Court of the Masorti Movement.