First, as to the word "Tu," pronounced "too" in the name "Tu B'Shevat." "Tu" is constructed from the Hebrew letters "tet" and "vav." "Tet" is the ninth letter in the Hebrew alphabet; "vav" is the sixth. Nine + Six = Fifteen. Also, "tet" is a consonant, making the "te" sound; "vav" is a vowel, which when sounded with the "shoorook" vocalization, sounds like "oo."
This is the Rosh HaShanah for trees. Again, as mentioned above in connection with the "First of Elul" Rosh HaShanah, the significance is primarily in connection with "Maasrot," tithing, or taking tenths. The dates of ripening of different agricultural species, say wheat, on one hand, and apples and oranges representing the "fruit-of-the-tree" group, are different. Also, since one may not calculate the "tenth" for a given year using produce from a different year, it is important to know the calendar definitions of ripening which apply to the various species.
Our Sages have designated the 15th of Shevat as the boundary, for trees, between one year and another, since most of the rains of the previous year, in the Land of Israel, have already fallen. A certain percentage of the fruit has reached the stage of "begun to ripen." This is defined as from the time of blossoming until the fruit has reached one third of its full growth. Fruit which have reached this stage are attributed to the previous year. Any new blossoming of fruit after this day is a result of the blessings of the new year.
Special Note: The earliest-ripening fruit is the "shekadiah," the "almond," in honor of which the following famous song was composed:
The Almond has blossomed
"VeShemesh Paz Zorachat"
And the Sun is Shining Brightly
"Tzipporim Me'Rosh kol Gag"
Birds from Every Roof-Top
"Mevasrot et Bo HaChag"
Welcome the Arrival of the Holiday
Although the 15th of Shevat is called Rosh HaShanah, the designation applies only to the matter of tithes that are due from fruit of the trees. Work is not prohibited, and there are no required festive meals, and no special prayers added to the regular prayer services. Nevertheless, the day is invested with a festive sense. Tachanun is not said. Eulogies are not delivered for the dead, and if it falls on Shabbat, Av Harachamim is not said (since Av Harachamim recalls the souls of the dead.) It is customary to eat a new fruit from the Land of Israel of which one had not yet partaken the present year, so that the "bracha" or "blessing" of SheHecheyanu may be said.
The reason for the festive mood of the Rosh Hashanah of trees is that the 15th of Shevat recalls the praise of the Land of Israel, for on this day the strength of the soil of the land is renewed. With reference to the fruits of the trees and the produce of the soil, the Torah praises the Land of Israel: "A land of wheat and barley and vines and fig trees and pomegranates, a land of olive trees and honey" (Devarim 8).
Another reason for the special observance of the 15th of Shevat is that the time of Rosh Hashanah for the trees is also a time of prayer and judgment concerning the trees. Whenever any of His creatures begins to grow, G-d surveys its entire future. So it is proper, at such a time, to pray that the new creature or being might prosper.
The Torah has compared Man to a tree of the field; hence this day also recalls the Divine judgment upon man. For such is the character of the people of Israel, that they rejoice on a day of judgment. Whatever the decision is, let all see that "there is a law and that there is a Judge." The Torah is the law, and G-d is the Judge.