The Hebrew word Tu is an abbreviation that means "fifteen". The Hebrew word Sh'vat is the name of a month. The prefix b' means "in". Thus the phrase Tu B'Sh'vat means "the fifteenth day in the month of Sh'vat".
Scripture requires that the first fruit of each harvest season be especially designated for God. This commandment required a decision about when the fruit tree harvest season began. Some trees bloom quite early or quite late compared to others. By legislating a certain day as the start of the annual fruit tree season, the Jewish leaders of the Second Temple period could prevent ambiguity: a tree's fruit could clearly be the last of the old season or the first of the new season.
Like many issues, the schools of Shamai and Hillel argued about the choice of day. Shamai favored the first day of the month of Sh'vat, and Hillel favored the fifteenth day of the month of Sh'vat. (The month was chosen because it was the middle of winter in Israel. Thus few, if any, trees were bearing fruit at that time.) When the Rabbis gained power, they affirmed Hillel's choice.
Scripture also requires that a person not eat of a young fruit tree for the tree's first three years, and on the fourth year its fruit is to be used for giving praise to God (Leviticus 19:23-25). The way this commandment is phrased makes clear that it is actually harvests being counted, even if some of the third or fourth harvest happens after the anniversary of the day on which a certain tree had been planted or had first sprouted. Thus Tu B'Sh'vat also became the "birthday of trees"—a tree's fruit was forbidden or set apart until Tu B'Sh'vat had passed a sufficient number of times.
In the Second Temple period this day was not celebrated as a holiday.
In the sixteenth century, Jewish mysticism flourished in Israel, and specifically in the city of Tzfat. Tu B'Sh'vat was here celebrated with a seder meal similar to the Pesach seder. Different fruits were eaten to symbolize different aspects of holiness, life, or fruitfulness. These traditions spread to Europe, and sixteenth-century Tu B'Sh'vat may be the earliest recorded European traditions linking nature's fruitfulness with humanity's fruitfulness and promoting ecological awareness.
The Jewish mystics did not standardize the interpretations they gave to fruits eaten on Tu B'Sh'vat. But certain categories of fruit became traditional: seven species of Israeli fruits mentioned in Deuteronomy 8:8 (figs, dates for date-honey, pomeganates, olives, grapes or raisins, wheat, and barley), nuts with shells or fruits with thick and inedible peels, fruits with inedible pits, and fruits with edible seeds. An eduring tradition was to postpone eating one of the seven species of Israeli fruits until Tu B'Sh'vat.
During the last hundred years, modern Zionism's efforts to restore the land of Israel have popularized donating money so trees may be planted in Israel on Tu B'Sh'vat. This custom has been so successful that the need it addreses has lost its sense of urgency, and many Diaspora Jews now plant trees in their local communities instead.
The holiday is not commanded in scripture, but can still be celebrated in beneficial ways.
Many Diaspora Jews eat fruit from Israel on Tu B'Sh'vat, to celebrate the return of the land to fruitfulness, and to express a longing to be in Israel.
Appropriate prayers include the Shehechyanu (thanking God for providing a joyous occasion) and the standard blessing over fruit. Appropriate messianic passages of scripture include Isaiah 4, Isaiah 27:2-6, Isaiah 65:17-25, and John 15:1-17.
Many Jewish communities host some sort of nature-related activity on Tu B'Sh'vat. These activities have been anything from tree planting to trail clearing to a recent campaign (in Portland, Oregon) to raise awareness of the pollution caused by letting cars idle.