This year the holiday of Tu B'Shevat occurs on February 3, 2007
The Mishnah says that according to Beis Shammai, the Rosh HaShanah (New Year) for trees is Rosh Chodesh Shevat (the first day of the month of Shevat). However, the students of Hillel rule that the New Year for trees is on the fifteenth of Shevat. Although in Western Astrology the zodiacal representation of Aquarius is the water bearer, in Kabbalah, the sign for the month of Shevat is the bucket of water itself. Based on the words of Rashi, Rabbi Eliyahu Kitov (1912-1976) explains that by Tu B'Shvat most of the rainy season's waters have already fallen, and the water collects in the wells. This signifies the end of the process needed to nurture floral growth. Tu B'Shvat is not only an important holiday for trees, but it even has meaning for people. This is because trees and fruits are used throughout Torah literature to serve as metaphors for humans.
In the realm of Halacha, the Shulchan Aruch (Rabbi Yosef Karo, 1488-1575) rules that the custom is not to recite the Tachanun supplications on Tu B'Shvat. The Magen Avrohom (Rabbi Abraham Abele Gombiner, 1633-1683) adds that on Tu B’Shvat there is a custom to increase our consumption of fruits. Rabbi Yehuda Dov Zinger writes that the custom is specifically to eat fruits from the land of Israel and/or fruits of the seven species for which the land of Israel is praised. He also writes that there is a custom to specifically eat fifteen different types of fruits and recite one of the fifteen songs of ascent between eating each of them. He records a custom in the name of Rabbi Chaim Pelagi (d. 1868) to learn a specific chapter from the Mishnah after each of the fifteen fruits: Eight of the chapters are from Tractate Peah, three from Bikkurim, and four are from Rosh HaShanah. Others have the custom of staying up all night learning Torah or reciting the Tikkun prepared by the Arizal (Rabbi Yitzchok Luria, 1534-1572), entitled, Pri Eitz Hadar, "A fruit from a beautiful tree", which is a reference to the fruit of the Esrog (Citron) tree.
The Satmar Rov, Rabbi Yoel Teitelbaum (1887-1979), asks why it is the custom to eat fruits on Tu B'Shvat, the New Year for trees, and while on Shavuos, the day on which the world's fruits for the year are judged the custom is to decorate houses and synagogues with trees. Logically, it should be the opposite. He explains that when a father is judged in the heavenly courts, in addition to considering his own actions, the court also examines the actions of his children, to see if the father raised the children properly. Similarly, when a child is judged, his father’s actions are also taken into account. Therefore, on Tu B’Shvat, the day when the trees are judged, the custom is to to give the trees more merits by performing extra mitzvos using the fruits (their “children”), while on Shavuos, the day when the fruits are judged, extra merits are acquired for the fruits by using the trees for Mitzvos.
In a similar fashion, the Talmud compares blessing a Talmudic scholar to blessing a tree. When one Amora (Rabbi from the time of the Talmud) requested a blessing from another, the latter answered with a parable likening the situation to a man who walks in the desert and comes across a tree. He eats from the tree, drinks from a nearby brook, and sits in the tree’s shade. Afterwards, he wants to thank the tree for having saved him from hunger, thirst, and brutal desert sun. However, he realizes that there is nothing with which he can bless the tree because its fruits are already sweet, a creek flows alongside it, and its shoots produce ample shade. Therefore, he blesses the tree that all of its fruits should produce trees which are similar to it. Similarly, a Talmudic scholar is already blessed with all possible blessings. Therefore, the second Talmudic scholar gave the first scholar the greatest possible blessing that he should father children who will follow in his path of greatness.
The Bnei Yissascher (Rabbi Tzvi Elimelech Shapiro of Dinov, 1785-1841) writes that when the Mishnah states that Tu B'Shvat is the Rosh Hashanah for "the tree" in the singular tense, instead of "trees" in the plural, the Mishnah was referring specifically to "the tree" mentioned in the Torah: namely, the Esrog (citron) tree. Based on this, he writes that there is a custom to pray on Tu B'Shvat that he should merit to be granted a beautiful Esrog fruit for use on the holiday of Succos. His great-great-grandson, the Munkatcher Rebbe (Rabbi Chaim Elazar Shapiro of Munkatch, 1871-1937), explained this concept in greater depth. The numerical value of Shevat is equivalent to the numerical value of Ish (which means "man"). Because the word “Esrog” is treated grammatically in Hebrew as if it was in the female tense, the Esrog is a representation of the feminine component in the world. The Talmud says that is the nature of men to actively seek out women, but the nature of women is not to seek men, therefore it serves to reason that during Shevat, the month of "man", one would pray (thereby actively seeking) for a beautiful Esrog, which represents the female element.
It was the custom of the Munkatcher Rebbe not to pickle or otherwise prepare his Esrog for consumption until the eve of Tu B'Shvat. In order to use the Esrog for another Mitzvah (after using it on Sukkos) of eating fruit on Tu B'Shvat, he dipped it in each of the the aforementioned seven species of fruits. The Belzer Rebbe also had the custom of eating from the seven species in varied forms: He drank beer for barley and wine for grapes, he ate bread for wheat, fish soaked in olive oil for olives, date honey, pomegranates, and figs. It was also his custom to say words of Torah at this meal. The Satmar Rebbe, however, refrained from speaking Torah and instead sang Psalm 96.
The Toldos Yitzchok, (Rabbi Yitzchok of Neshchiz) writes that the reason that the Talmud says that one must begin studying the laws of a holiday thirty days before the holiday is because that is when the mystical influences of the holiday begin. Therefore, since Tu B'Shvat is thirty days before the holiday of Purim (in a regular non-leap year), the two must have some connection. He says that Shevat can mean either "rod" or "throwing." Concerning Purim, the Talmud explains that the Jews only sinned outwardly by bowing to the idol of Haman but really they still believed in HaShem. So too HaShem only "outwardly" wanted to punish the Jews by making them feel as if they were going to be destroyed, He had no intention of allowing such a catastrophe to actually occur. Based on this, the Toldos Yitzchok explains that in actuality Shevat means both "rod" and "throwing". For the entire year until Shevat, HaShem holds a "rod" as if to threaten the world that He will destroy them if they do not act as they should. Once Shevat arrives, He "throws" away the stick, and reveals that He was merely trying to scare everyone into proper behavior. This is comparable to a father who rouses fear in his son by threatening him with a rod. However, the father does not intend to actually harm his dear son—he planned the ruse merely to ensure that the son act appropriately.
In the opening words of Psalms, King David, the psalmist, utilizes a simile to describe a righteous Torah Jew. He writes, "He shall be like a deeply rooted tree on the brooks of water…." In this, King David compares the upright Jew to an upright tree. Two other prophets also use this comparison: Isaiah said, "Just like the days of a tree, so too shall be the days of My people" and Jeremiah said, "He will be like a tree planted near the water". HaShem Himself has compared righteous Jews to trees when He said, "Man is like the tree of a field." Many Jewish customs developed because of this association between people and trees. There is a law that one may not derive any benefit from a tree’s fruits while the tree is within its first three years (regarding which Tu B'Shvat is considered the beginning of a new year). Since people are compared to trees—more specifically to fruit trees—a custom developed, based on the Arizal’s Kabbalistic teachings, to not cut a baby boy's hair until he reaches the age of three years.
Rabbeinu Bachaya explains that people are compared to trees because their sustenance comes from trees. King Solomon said, "Torah is a tree of life for all who those grasp it." Rabbi Akiva said that a Jew without a Torah is like a fish out of water. The Prophet Isaiah invited all those who were thirsty to go to the waters; The Talmud assumes that this "water" refers to Torah, and those who were thirsty were seeking its wisdom. Indeed, the Talmud assumes that when King David desired waters from the well in Beis Lechem, he wished to clarify a halachik question. Just as water is the sustenance, from which a tree feeds and grows, so too Torah is the sustenance from which a Jew lives and thrives. The Torah is the water from which man—the tree—grows.
In addition to requiring water in order to be properly nurtured, a tree also requires sunlight—fire. Similarly, in order to be successfully developed, a Jew needs both the depths of the “waters of Torah” and the passionate and fiery debates in Torah—the fire. While Isaiah compared men to trees because both require water, Jeremiah asked rhetorically in the name of HaShem, "Are My words not like fire?" The Talmud explains that just as a fire cannot burn alone, so too the words of Torah cannot prevail in isolation and just as a fire is built from many logs, so too the words of Torah survive only through the minds of the many. Interestingly, Moses began to elucidate the Torah in great depth for the Jewish people (before his death, a month and a week later) in the beginning of the month of Shevat.
A custom popularized by contemporary society is to plant trees (especially in Israel) on Tu B'Shvat. Although this custom lacks a clear source within Rabbinic literature, one can conjecture that this custom developed from Az Yashir which the Jews famously sang after the splitting of the Red Sea in Az Yashir (which is always read on Shabbos Shira, the week of Tu B’Shvat). One line of this song is "You shall bring them and plant them on the mountain of Your heritage, the foundation of Your dwelling-place that has been prepared by You, HaShem—the Sanctuary of HaShem which Your hands established." Perhaps the source of the custom to plant trees in Israel stems from this concept of planting something in Eretz Yisroel. However, others explain that the planting mentioned in this passage refers not to physically planting greenery in the Land of Israel, but rather to the building of the Holy Temple in Jerusalem on Mount Zion—upon the mountain of HaShem's inheritance. Regarding the conclusion of this verse the Talmud says, "Great is the Holy Temple which is written in between two instances of HaShem's name." May it be the will of HaShem that we merit to see the building of the Holy Temple, speedily and in our days and return to our roots in Jerusalem: Amen.
 Rosh HaShanah 2a
 To Rosh HaShanah 14b
 In Sefer HaToda'ah’s description of Shevat
 See also Siddur Ya’avetz, Shaar HaGay
 Shulchan Aruch, Orach Chaim §131:6
 However, it has been recorded in many places that the French Jewish communities of Mainz and Worms did say Tachanun on Tu B'Shvat
 Magen Avrohom ad loc., see also Mishnah Berurah and Beiur Heitiv ad loc. in the name of Tikkun Yissaschar.
 Ziv HaMinhagim
 They are: Wheat, barley, grape, fig, pomegranate, olive, and date honey
 Psalms 120-134
 Mahari Tab page 143
 Rosh HaShanah 1:2
 See Mishnah Berurah, §494:10
 A similar explanation is found in the writings of Rabbi Isaiah Horowitz (1565-1630) who explained (in Shnei Luchos HaBris) why Leviticus 26:42 lists the three forefathers in reverse chronological order instead of the usual chronological listing
 Ta'anis 5b-6a
 Bnei Yissaschar to Tu B'Shvat
 Rosh HaShanah 1:1
 Although others, such as Rabbi Tzadok HaKohen Rabinowitz of Lublin (1823-1900), understand that the singular expression of "the tree" refers to the Tree of Life (Eitz HaChaim) in the Garden of Eden. See Pri Tzadik who elaborates on this concept at great length.
 Leviticus 23:40
 Sha'ar Yissaschar, Tu B'Shvat
 Kiddushin 2b
 Darchei Chaim V'Sholom
 Minhagei Belzah
 Machzor Divrei Yoel to Tu B'Shvat
 Toldos Yitzchok on Tu B'Shvat
 Megillah 29b
 Megillah 12a
 Psalms 1:3
 Isaiah 65:22
 Jeremiah 17:8
 Deuteronomy 20:19, see Ta’anis 7a which explains that this refers specifically to a Talmid Chacham.
 See Tractate Orlah
 Rabbeinu Bachaya to Deuteronomy 20:19
 Proverbs 3:18
 Brachos 61b
 Isaiah 55:1
 Bava Kamma 17a, Bava Kamma 82a, Avoda Zarah 5b
 See Samuel II 23:15
 Jeremiah 23:29
 Taanis 7a
 See Deuteronomy 1:3. See also Zechariah 1:7 that the eleventh month is the month of Shevat.
 Exodus 15:17
 Brachos 33a