Talmud
Tu Bishvat appears in the Mishnah in Tractate Rosh Hashanah as one of the four new years in the Jewish calendar. The discussion of when the New Year occurs was a source of debate among the rabbis: "And there are four new year dates: - The first of Nisan - new year for kings and festivals - The first of Elul - new year for animal tithes. Rabbi Elazar and Rabbi Shimon say: the first of Tishrei. - The first of Tishrei- new year for calculation of the calendar, sabbatical years and jubilees, for planting and sowing - The first of Shevat, according to the school of Shamai; The school of Hillel say: the fifteenth of Shevat" (Rosh Hashana:2a)
The rabbis of the Talmud ruled in favor of Hillel on this issue. Thus the 15th of Shevat became the date for calculating when the agricultural cycle began or ended for the purpose of biblical tithes.
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