(א) אַרְבָּעָה רָאשֵׁי שָׁנִים הֵם. בְּאֶחָד בְּנִיסָן רֹאשׁ הַשָּׁנָה לַמְּלָכִים וְלָרְגָלִים. בְּאֶחָד בֶּאֱלוּל רֹאשׁ הַשָּׁנָה לְמַעְשַׂר בְּהֵמָה. רַבִּי אֶלְעָזָר וְרַבִּי שִׁמְעוֹן אוֹמְרִים, בְּאֶחָד בְּתִשְׁרֵי. בְּאֶחָד בְּתִשְׁרֵי רֹאשׁ הַשָּׁנָה לַשָּׁנִים וְלַשְּׁמִטִּין וְלַיּוֹבְלוֹת, לַנְּטִיעָה וְלַיְרָקוֹת. בְּאֶחָד בִּשְׁבָט, רֹאשׁ הַשָּׁנָה לָאִילָן, כְּדִבְרֵי בֵית שַׁמַּאי. בֵּית הִלֵּל אוֹמְרִים, בַּחֲמִשָּׁה עָשָׂר בּוֹ:
(1) There are four new years:The first of Nisan is the new year for kings and for festivals. The first of Elul is the new year for the tithe of beasts. Rabbi Elazar and Rabbi Shimon say: the first of Tishri. The first of Tishri is the new year for years, for shmitta and jubilee years, for planting and for [tithe of] vegetables. The first of Shevat is the new year for trees, according to the words of Bet Shammai. Bet Hillel says: on the fifteenth of that month.
The Kabbalists carried this relationship of Tu B'shvat and Rosh HaShanah a step further. For them, trees were a symbol of humans, as it says, "for man is like the tree of the field." (Deut. 20:19) In line with their general concern for Tikkun Olam---spiritually repairing the world---the Kabbalists regarded eating a variety of fruits on Tu B'Shvat as a way of improving our spiritual selves. More specifically, they believed that eating fruit was a way of expiating the first sin---eating the fruit of the tree of knowledge in the Garden of Eden. Similarly, trees were symbolic of THE tree--the Tree of Life, which carries divine goodness and blessing into the world. To encourage this flow and to effect Tikkun Olam, the Kabbalists of Sefat (16th century) created a Tu B'shvat seder loosely modeled after the Passover Seder.
(א) שלא לאכול ערלה - שלא נאכל מפרות האילן תוך זמן ערלתו (ספרא קדושים ג ג) והן שלש שנים ראשונות לנטיעתו, ואחד הנוטע נטיעה או יחור מן האילן, שנאמר (ויקרא יט כג) שלש שנים יהיה לכם ערלים לא יאכל.
(1) To not eat orlah: That we not eat from the fruits of a tree during the time of its orlah, which is the first three years from its being planted. And it is the same if one planted a sapling or a sprig from a tree, as it is stated (Leviticus 19:23) "three years shall it be orlah for you, it shall not be eaten."
§ Rav Mesharshiyya raised an objection against those who claim that Beit Shammai did not act in accordance with their opinion. It is taught in a mishna: There was an incident involving Rabbi Akiva, who collected an etrog on the first of the month of Shevat and performed with it two tenths. In other words, he separated two tithes from the fruit, as though it belonged to two different tithing years. He removed both the second tithe and the poor man’s tithe at the same time, two tithes that should not be separated in the same year. The mishna explains: One tithe he removed in accordance with the statement of Beit Shammai, who hold that the new year for trees occurs on the first of Shevat, which means that the etrog required tithing according to the regulations of the upcoming year. And one tithe was in accordance with the statement of Beit Hillel, who hold the new year for trees is on the fifteenth of Shevat, and therefore the etrog required tithing in accordance with the previous year. The Gemara infers: Conclude from here that Beit Shammai did act in accordance with their opinion, as Rabbi Akiva took care to act in accordance with the ruling of Beit Shammai.
Shimon b. Yohai said, "If you are holding a sapling in your hand, and someone says that the Messiah has drawn near, first plant the sapling, and then go and greet the Messiah.”