Question: Why is Tu B’Shevat, known as the New Year for Trees, in the middle of the month and not at the beginning of the month – like all other New Years?
Answer: The first mishnah in Tractate Rosh Hashanah lists the various New Years. Each of them, like you write, falls on the first of the month except for Tu B’Shevat.
There are exceptions, however. For example, the Gemara (ibid., 4a) asks regarding the New Year for festivals (which is also used to reckon the years of a king’s reign): “How can the New Year for the festivals be on the first of Nissan, when surely it is on the 15th of Nissan?” The Gemara answers that the mishnah means to say that the festival, Pesach, that occurs in the first month of the year marks the New Year for festivals. The New Year itself, though, starts on the 15th.
Two additional New Years – not enumerated in our mishnah – also do not fall on the first of the month. The New Year relating to the omer – the sacrifice that permitted one to partake of newly harvested grains of the five species throughout the land – occurs on the 16th of Nissan, and the New Year for the shetei halechem (two loaves) – permitting the use of flour from newly harvested grains for meal-offerings in the Beit Hamikdash – occurs on the 6th of Sivan. The Gemara explains that the mishnah does not list these two New Years because they start during the day rather than the previous night.
Thus, we see that Tu B’Shevat is not that unique. However, perhaps it appears to be so because it is the only New Year listed in the mishnah that does not occur on the first of the month (in some sense of the word) according to Beit Hillel, whose ruling we follow. Beit Hillel states that sufficient rain has fallen by the 15th of Shevat, enabling trees to blossom. We therefore set the New Year for trees at that point.
For a more esoteric understanding of the significance of the 15th of Shevat as the New Year for trees, we turn to the author of the chassidic work, Ohev Yisrael by Rabbi Avraham Yehoshua Heschel of Apta, who discusses this matter. We glean from his words:
“Regarding Tu B’Shevat, we must know and understand why it is stated specifically there (in the mishnah), ‘The New Year of the Tree, according to Beit Hillel, is on the 15th of Shevat, while according to Beit Shammai it is on the first of Shevat.’ It is also important to understand the reference to ‘tree’ in the singular, when it should have stated [the New Year of the] Trees, in the plural.
“We must answer that it states in the Torah (Deuteronomy 20:19), ‘For man is the tree of the field.’ [Here the author is alluding to the interpretation in Gemara Ta'anit 7a.] Just as the tree possesses roots, branches, leaves and fruit, so does the Jew possess all these because of his good deeds. How are these drawn to man? They stem from their source, the root of the Jewish soul, which is the Holy Tree – the Tree of Life under which all Creation’s animals and birds of the skies seek shelter. It is the tree that is blessed so that all its shoots are like it.
“The word ilan [tree in Hebrew] is numerically equivalent to the two Holy Names, Havaya and Adnut (their combined total is 91). This is in accordance with the hidden meaning of ‘Tzaddik katamar yifrach – A righteous man shall blossom as the date tree…’ (Psalms 93:13). Just as the palm tree has the means of propagating itself, so, too, do the righteous bring forth those that will propagate themselves.”
Rabbi Avraham Yehoshua Heschel continues with a citation from Tractate Rosh Hashanah (10b-11a). R. Yehoshua claims the world was created in Nissan, but R. Eliezer argues it was created in Tishrei. (These two months both launch the beginning of a different half of the year.) Rabbi Heschel points out that both these statements are “the living words of G-d” – both are true in some sense. He explains: “On [the first of] Tishrei the thought came to His mind to create the world, as the paytan notes [in our Rosh Hashanah liturgy], ‘Hayom harat olam – Today You have conceived the world.’ However, the actual creation was in Nissan.”
He then offers a lengthy explanation, comparing the tree to the original Creation by presenting the month of Shevat as a microcosm of the 12 months of the year and dividing Shevat into two parts. He compares the first half of the month to the conception of trees – the part of creation that is hidden. This is actually the essence of Beit Shammai‘s opinion, whose rulings hold sway in the Heavenly Court. Beit Hillel, on the other hand, represents that which is revealed – like the blossoming of trees. For the most part, blossoms appear on the first day of the second half of the month – Tu B’Shevat.