The Mishna (2a) brought a disagreement between Beit Shammai and Beit Hillel as to whether the new year for trees begins on the first day of Shevat or on the fifteenth day of that month. Thus, fruits that bud before this date will belong to the previous year with regard to the rules of terumot and ma’asrot; if they bud afterwards, then they will belong to the subsequent year
The Mishna taught: On the first of Shevat is the new year for trees, according to the statement of Beit Shammai. The Gemara asks: What is the reason that the new year for trees was set on this date? Rabbi Elazar said that Rabbi Oshaya said: The reason is since by that time most of the year’s rains have already fallen, and most of the season, i.e., winter, is yet to come, as it continues until the spring equinox, which usually occurs in Nisan.
Since Tu bi-Shevat is based on the lunar calendar, it can fluctuate anywhere from January 17 until February 14, although in most years it falls out at the end of January or beginning of February. Winter runs from December 22 through March 21, so most years the majority of winter occurs after Tu bi-Shevat. As far as the rainy season in Israel is concerned, there is evidence that suggests that in ancient times rains fell earlier in the year than they do today. Nevertheless, even today more than 50% of annual rainfall takes place before February, so usually most rainfall comes prior to Tu bi-Shevat.
How do Beit Shammai and Beit Hillel arrive at the dates that they choose to identify as the new year?
The Me’iri suggests that this date is the middle of the rainy period. Perhaps Beit Shammai is reluctant to divide a month in half (the Talmud Yerushalmi teaches that the months are established as single units based on the passage in Shemot 12:2 – le-hodshei ha-shanah – “of the months of the year”). Beit Hillel, on the other hand, is less concerned with keeping every month as a single unit, and is more concerned with the relationship between the trees and the natural cycle of the seasons.