Tu Bishvat (Hebrew: ט״ו בשבט) is a minor Jewish holiday, occurring on the 15th day of the Hebrew month of Shevat (in 2011 this occurred from sunset on January 19 through sunset on January 20). It is also called "The New Year of the Trees" or (Hebrew: ראש השנה לאילנות, Rosh HaShanah La'Ilanot). Tu Bishvat is one of four "New Years" mentioned in the Mishnah. This year, it will fall upon the dusk of February 7 through the dusk of February 8, 2012.
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:1a)
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.
In the Middle Ages, Tu Bishvat was celebrated with a feast of fruits in keeping with the Mishnaic description of the holiday as a "New Year." In the 16th century, the kabbalist Rabbi Yitzchak Luria of Safed and his disciples instituted a Tu Bishvat seder in which the fruits and trees of the Land of Israel were given symbolic meaning. The main idea was that eating ten specific fruits and drinking four cups of wine in a specific order while reciting the appropriate blessings would bring human beings, and the world, closer to spiritual perfection.
In Israel, the kabbalistic Tu Bishvat seder has been revived, and is now celebrated by many Jews, religious and secular. Special haggadot have been written for this purpose.