The original significance of the Fifteenth of Shevat [“Tu BiShvat”] was to demarcate the agricultural year to which a fruit belongs, since tithes and other religious laws apply separately to the crops of each year. The Mishnah situated the dividing line for trees in the month of Shevat which occurs towards the middle of the Israeli rainy season. The School of Hillel, whose position was accepted as normative, placed it on the fifteenth day of that month.
As regards the reason for selecting this date, Rabbi Hoshaiah in the Talmud explained that by this stage most of the year’s rain can be assumed to have fallen; hence the older fruit derived their sustenance from the previous year, whereas subsequent fruits belong to the coming year. Rashi explained that the trees’ reliance on the new water is recognizable by the flow of their sap.
Commentators differ whether the operative consideration is the number of days between the onset and termination of the rainy season, the volume of rainfall, or the change in temperature. All of the above positions accept the premise that the relevant transition occurs on the fifteenth of Shevat
But wait. The months on the Jewish calendar are calculated by a lunar cycle in which twelve months of twenty-nine or thirty days add up to 354 days—whereas the natural seasons that govern rainfall, plant growth and temperature are based on the 365-day solar year! Indeed, talmudic sages raised the question of whether “Shevat” was being used here in its normal sense as a lunar month, or as a designation for a date one month into the solar winter season (known as the “teḳufah of Tevet”). They concluded that the reference was to the lunar month of that name. Indeed the new year for trees would commence on the same date even in a leap year, when an extra month is inserted and the lunar year is seriously out of sync with the solar teḳufah.
A Babylonian Gaon observed that Shevat marks the time when trees awaken from their winter dormancy, beginning to soak up fluids and come alive. This stage, which would qualify as an appropriate “new year of the trees,” occurs around the midpoint of “Shevat” of the solar cycle (around January 30).
The Tosafot expressed astonishment at the Talmud’s preference for a lunar date even though the ripening of the fruit follows the sun. It is perhaps typical of their thinking that they relied less on empirical observation of the botanical facts than on scriptural prooftexts. They invoked the words of Deuteronomy about “the precious fruits brought forth by the sun”; but noted that the same text goes on to speak of “the precious things put forth by the moon.” Ultimately they conceded that “in most calendar-related matters Israel reckons according to the moon.”
Rabbi Moses Sofer of Pressburg (“Ḥatam Sofer”) found this explanation inadequate. He cited Rashi’s comment to Deuteronomy that the influence of the moon is limited to vegetables that grow from the earth like cucumbers and gourds; whereas trees follow the rhythms of the sun. It is thus quite possible that the date of the fifteenth of Shevat might arrive—as in a leap year—before the tree’s sap has begun to flow.
The Talmud taught elsewhere that “the year follows the moon,” which Rashi explained in the sense that we follow the months as they would have been if the year had not been intercalated. For the sake of convenience we call this month Shevat, but the decisive date should be determined by the climatic conditions.
“If this is so,” writes Rabbi Sofer, “then what difference does it make whether or not in most years it is designated as Shevat, seeing that in any case the chill of Tevet persists during Shevat, so that there is no sap in the trees and they are not ripening —and yet the tithing follows the year of the ripening! Under the circumstances, what grounds are there for declaring the new year in Shevat?” And even if (as the Tosafot asserted) Jews follow lunar chronology for other purposes, that should not warrant a violation of Tu BiShvat’s botanical basis.
Several authorities cited a passage from the Jerusalem Talmud in which Rabbi Hoshayah admonished the witnesses to the sighting of the new moon by reminding them that their role in determining the dates for the beginnings of months had tangible legal consequences. This was true not only in civil matters (such as defining rent periods), but even for capital cases; for by determining a person’s birthday, they might also be defining whether an offender is an adult or a minor. In the latter instance, even though the formal ages of twelve years for a female or thirteen for a male are supposed to serve as indications that the person has matured physically—in practice the law subordinates the physiological processes to the authority of the court charged with regulating the calendar.
In support of that principle Rabbi Abin expounded the words of Psalms: “unto God that performeth [gomer] all things for me,” which could be interpreted to mean “God completes things according to me,” in the sense that the Almighty sometimes defers to the decisions of my [=human] authority.” Accordingly Rabbi Solomon Ibn Adret concluded, “whatever an earthly court declares is confirmed by the celestial court.”
Scholars like Rabbis Judah Mintz and Shabbetai Cohen inferred from this that the holy Torah can override scientific evidence in the realms of physiology, botany or climate—or at least warrant reinterpretations of the empirical data.
The Ḥatam Sofer’s theological position resonated with some later Jewish traditionalists, and perhaps we should blame him and his followers for the outlook, current in some communities, that scientific standards of public health must yield to the religious benefits of superspreader weddings, funerals or yeshivah classes.
However, nature—as we are often reminded—does not like to be trifled with.