Tonight is TU BISHBAT (ט״ו בשבט), which means, the 15th day of the month of Shebat. In Hebrew the letter ט/T represents the number 9, because is the ninth letter of the alphabet, and the letter ו/U represents the number 6, because it is the sixth letter of the Hebrew alphabet. Together. 9 + 6 =15. Usually, the numbers after 10 would be written with the letter YOD/י, the tenth letter of the alphabet, followed by a numeral. Thus, if I want to say in Hebrew “14” I will write the letter YOD and the letter DALET י״ד = 14. But since the combination of the letters YOD and the letter HE (10+5) would result in the abbreviated name of God, in order to write “15” we use 9+ 6 instead pf 10+5.
The 15th of Shebat is the “new year of the trees” (Rosh haShana laIlanot). In the fifteenth of Shebat the trees are considered one year older. This in an important factor for many Mitsvot connected to agriculture, for example, ‘orla: the prohibition to eat the fruits of a new tree during the first three years after it was planted. The age of a tree does not increase on the date it was planted (the tree’s “birthday”, rather, all trees become Halakhically one year older on T”U BISHBAT (Tomorrow, BH, I will explain in more detail the importance of determining the age of the trees and the application of this factor in a number of Biblical commandments).
Now, in the Tora or in the Mishna there is no indication about “celebrating” the 15th day of Shebat. It is not considered a holiday, nor a commemoration of any historical event, and there are no prohibitions, no ritual ceremonies prescribed for this day. However, the custom since many centuries ago is to have a special se’uda (a meal or a plate) of fruits in TU BISHBAT to remember the fruits of erets Israel.
There are no Halakhic instructions for the celebration of this se’uda, but here are several customs. In many communities it is customary to eat at night (tonight) and/or during the day all type of fruits, and especially the seven fruits by which the Land of Israel was praised in the Tora. Israel was described as “…a land of wheat and barley, grape and fig and pomegranate; a land of olives and honey (=dates)” (Deut. 8:8).
Many communities have the custom to organize a formal Seder of TU BISHBAT saying the blessing ha’ets for the fruits of the tree, and the correspondent blessings for whatever else is eaten, like wheat, barley (normally mezonot), etc. and each berakha fruit is followed by a prayer, yehi ratson.
Many consider a special zekhut to have for this Seder actual fruits from the Land of Israel and say berakha for them.
No special prayers are added to the regular services of TU BISHBAT. Tahanun, however, is not recited in TU Bishbat.
For a more detailed explanation of the history of TU BISHBAT, and particularly for understanding the view and practices of the Hakhme haKabbala on TU BISHBAT, read this beautiful article written by Mr Joseph Mosseri. See here