Tu Bishvat is a minor Jewish holiday, occurring on the 15th day of the Hebrew month of Shevat (in 2013, Tu Bishvat started from sunset on 25 January and finished at nightfall on 26 January). It is also called "Rosh HaShanah La'Ilanot" (Hebrew: ראש השנה לאילנות‎), literally "New Year of the Trees." In contemporary Israel the day is celebrated as an ecological awareness day and trees are planted in celebration. 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 the beginning of the agricultural cycle for the purpose of biblical tithes. On Tu Bishvat 1890, Rabbi Ze'ev Yavetz, one of the founders of the Mizrachi movement, took his students to plant trees in the agricultural colony of Zichron Yaakov. This custom was adopted in 1908 by the Jewish Teachers Union and later by the Jewish National Fund (Keren HaKayemet L’Israel), established in 1901 to oversee land reclamation and afforestation of the Land of Israel. In the early 20th century, the Jewish National Fund devoted the day to planting eucalyptus trees to stop the plague of malaria in the Hula Valley; today the Fund schedules major tree-planting events in large forests every Tu Bishvat. Over a million Israelis take part in the Jewish National Fund's Tu Bishvat tree-planting activities. In keeping with the idea of Tu Bishvat marking the revival of nature, many of Israel's major institutions have chosen this day for their inauguration. The cornerstone-laying of the Hebrew University of Jerusalem took place on Tu Bishvat 1918; the Technion in Haifa, on Tu Bishvat 1925; and the Knesset, on Tu Bishvat 1949. Tu Bishvat is the Israeli Arbor Day, and it is often referred to by that name in international media. Ecological organizations in Israel and the diaspora have adopted the holiday to further environmental-awareness programs. On Israeli kibbutzim, Tu Bishvat is celebrated as an agricultural holiday.

Source: Wikipedia