According to Sefer Yetzirah, each month of the Jewish year has a letter of the Hebrew alphabet, a zodiac sign, one of the twelve tribes of Israel, a sense, and a controlling limb of the body that correspond to it.
Tevet is the tenth of the twelve months of the Jewish calendar.
Tevet begins with the last days of Chanukah (which climax with its culminating, eighth day--Zot Chanukah). Its tenth day--the tenth day of the tenth month ("the tenth shall be holy to G-d")--is a fast day, in commemoration of the siege of Jerusalem, the beginning of the destruction of the Temple.
The four fast-days which commemorate the destruction of the Temple are (in the order of the year) the 17th of Tamuz (the 4th month), the 9th of Av (the 5th month), the 3rd of Tishrei (the 7th month), and the 10th of Tevet (the 10th month). Of these four days (in their respective months) the prophet says: "the fast of the fourth [month], and the fast of the fifth, and the fast of the seventh, and the fast of the tenth shall [in the future] be for the House of Judah for joy and happiness and holidays."
The sum of the four numbers--4, 5 7, 10--is 26, the value of G-d's ineffable Name of mercy, Havayah. (The four days of the four months are 17 [of Tamuz], 9 [of Av], 3 [of Tishrei], and 10 [of Tevet]. 17 plus 9 = 26; 3 plus 10 = 13, the value of the word echad ["one"]. 26 and 13 are thus the numerical value of Havayah echad ["Havayah is one"]. 26 plus 13 = 39. 26 [the value of the months] plus 39 [the value of the days] = 65 = Adnut).
The four numbers possess an orderly numerical progression, with finite differences of 1, 2, 3. The following three numbers in the progression--14, 19, 25--equal 58--chen ("grace"). Together with 26--Havayah--the first 7 numbers of the progression ("all sevens are dear") equal Chanoch (whose name, from the word for "education" and "initiation," is an acronym for "the grace of Havayah"), the seventh ("dear") generation from Adam.
All of the fast days, when observed properly, draw down Divine grace from the ultimate source of mercy, G-d's Name Havayah. The epitome of this process (in the order of the year, as in the words of the prophet quoted above) is on the tenth of Tevet (in the secret of "the end [final fast-day of the year] is wedged in the beginning [of the events which led to the destruction]"). By the Divine grace, the third, eternal Temple is built, first in the heart of Israel, thereafter to become physically manifest on earth.
The letter ayin means "eye." The month of Tevet is the month of the rectification and nullification of the "evil eye." The word Tevet itself comes from tov, "good," referring to tov ayin, "the goodly eye" (the source of the power of blessing, as it is said: "the goodly eye shall bless"). This rectification begins with the gazing at the Chanukah candles (especially when they are complete on the eighth day).
All destructive process begins with the "evil eye" of hatred, the hatred of the profane to the holy (the secret of ten, the holy number, as mentioned above). From hatred comes anger, the fire of destruction. The middle letter of ka'as, "anger," is the letter ayin. The negative ka'as must first be rectified to its positive correspondent, as will be explained presently.
Our sages teach us that at the age of ten (an allusion to the tenth month, the level of ten in general) a child "jumps like a goat." (Midrash Kohelet) The playful nature of jumping up and down "like a goat" reflects an important stage in the growing-up process. The month of Tevet, the month of the tribe of Dan, relates to the growing-up process, from a state of immaturity to a state of maturity.
Immaturity is characterized by the "evil eye," while maturity is characterized by the "good eye." Gedi = 17 = tov, "good" (the "good eye"). One must play (and jump up and down like a goat) in order to rectify and sweeten the anger latent in ones animal soul.
The tribe of Dan represents the initial state of immaturity in the soul that "grows-up" during the month of Tevet. Dan means "to judge." Initially, he judges reality and others critically, with severe judgment (the "evil eye"). This is the nature of one who is spiritually immature. Dan is likened to a snake, who bites with the venom of anger. The "evil eye" is the eye of the snake.
The rectification of Dan is his engaging in the battle of holy anger against evil anger. Our sages teach us that only one from the soul-root of Dan can spontaneously jump up and kill the evil snake--"one like him, killed him."
Nachash ("snake") = 358 = Mashiach. The holy power of Dan reflects a spark of Mashiach. In the Zohar we are taught that the commander-in-chief of the army of Mashiach will come from the tribe of Dan.
The sense of holy anger, or righteous indignation (the rectification of the month of Tevet) is the ability of the soul to arouse one's good inclination to become angry at one's evil inclination. This our sages teach us in their commentary on the verse in Psalms: "Be angry and don't sin."
Positive anger expresses the deepest care and concern of the soul that reality become good. Though even to this anger there is a certain element of immaturity (for absolute maturity, that of the Creator of reality, sees [with the ayin of Tevet] all as good), nonetheless, of it is said: "for Israel is [behaves as] a lad, and [therefore] I [G-d] love him."
In Chassidut we are taught that one must direct his left ("evil") eye towards himself (with the holy anger of his innate good against his innate evil), to lower and subdue his ego, while simultaneously directing his right ("good") eye towards outer reality (by which power he helps reality perfect itself).
The sages teach us that "the liver is angry." The function of the liver [kaved, in Hebrew] is to purify the blood with which it is saturated. In Kabbalah, the liver corresponds to the primordial snake, whose rectification is personified by Dan. (The three "rulers" of the body and soul are the brain, the heart, and the liver, which correspond to Adam, Eve, and the snake, respectively).
The snake, in Kabbalah, represents the initial state of immaturity of the soul, as characterized by the unrectified attribute of anger. The venom of the snake is hot (see above, the month of Cheshvan), like the fire of anger. When converted to the good, the fire (and blood of the liver) serve to warm the cold month of Tevet.
Kaved = 26 = Havayah. This reflects the secret mentioned above, that the sum of the four months which "fast" over the destruction of the Temple (by the venom of the primordial snake), which culminate in Tevet, together equal 26. By fasting over the destruction one rectifies one's liver--mellows one's anger--and thereby "sweetens" the anger of G-d (with Israel, the cause of the destruction) and arouses the mercy of Havayah to rebuild the Temple.