I
The first of Shevat is the new year for the trees, according to Beis Shammai. Beis Hillel says the new year is on the fifteenth of Shevat (Mishna Rosh Hashana 1:1). The Gemara (14a) explains that most of the year's rain has already ended by Rosh Chodesh Shevat. Beis Hillel's date is Tu B'shevat because that is when the trees actually blossom (Tosfos, Meiri).
This dispute may be related to two others in which (according to Rav Zevin - Le'or Hahalacha p. 304) Beis Shammai focuses on the potential and Beis Hille on the actual. On Chanukah, Beis Shammai rules that one lights eight candles on the first day (Shabbos 21b), since, at the time of the miracle, the oil had the potential to last for eight days. Each night, as the potential decreases, we light one less candle. Beis Hillel's accepted practice is to increase from one to eight candles, as the original miracle increased, in actuality, each night.
At havdala, Beis Shammai's beracha on the candle is "shebara me'or aish" (Berachos 51b). The past tense and singular usage refer to the primordial monochromatic fire (Gra), which represents the potential for all future fires. Beis Hillel's accepted beracha, "borai me'orai ha'aish ", alludes to the actual multicolored fires of today.
Similarly, Beis Shammai ushers in a new year for trees when the majority of the season's rain, which gives the tree potential to bear fruit, has fallen. Beis Hillel delays the new year until Tu B'shevat , when the trees actually blossom.
II
The respective dates of Beis Shammai and Beis Hillel represent climatological and agricultural milestones. Yet the themes of potential and actual recur within the lunar cycle.
On Rosh Chodesh , the moon is nearly invisible, but it has the potential to grow for the next two weeks. On the fifteenth the moon reaches the fullness of its potential and its greatest actual size. Beis Shammai's date emphasizes potential, while Beis Hillel's focuses on the actual.
When the moon "sinned", Hashem decreed that it become smaller (Chulin 60b). According to the Rashba (cited by Rabbeinu Bachya, Bamidbar 28:15), the moon was always the smaller luminary, but its size, like the sun's, remained constant. The punishment of becoming smaller refers to the moon's waxing and waning each month.
On Rosh Chodesh we are required to offer a chatas Lashem , as an atonement for Hashem's making the moon smaller (ibid.). We need atonement for not fulfilling our full potential, a shortcoming symbolized by the moon which is almost never full. The challenge of man, which will be fully met only at the end of days when the moon will no longer be diminished each month, is to utilize his potential to the maximum.
III
When Hashem first appeared to Moshe Rabbeinu, He said "Moshe Moshe" (Shemos 3:4). In contrast to "Avraham | Avraham" (Breishis 22:11) and "Yaakov | Yaakov"(ibid. 46:2), where a line is drawn between the two mentions of the name, there is no line in "Moshe Moshe" (Shemos Raba 2:6).
The first name refers to the ideal person, his essence in the upper world. The second refers to the real person in this world. Only Moshe Rabbeinu realized his full potential so that no line divided between the two mentions of his name (Ruach Chaim , Avos 1:1).
Moshe merited the unique zechus of receiving the Torah because of his unique humility (Bamidbar 12:3), which surpassed that of Avraham (Chulin 89a). Moshe viewed himself as if he didn't exist, and therefore his body did not prevent his soul from realizing its full potential (ibid.).
Perhaps the special status of Beis Hillel is attributable to a similar quality. Normative halacha follows Beis Hillel because they were humble, to teach you that Hashem praises one who lowers himself (Eruvin 13b).
Even though Beis Shammai were sharper (Yevamos 14a), and thus had greater potential, Beis Hillel were greater in actuality. Their fuller realization of their potential, like that of Moshe Rabbeinu, resulted from their humility. This mirrors the point of halachic contention regarding Chanukah, havdala , and the new year for trees in Shevat, in which Beis Shammai follows the potential, and Beis Hillel the actual.
Every person can be a tzadik like Moshe Rabbeinu (Rambam, Teshuva 5:2). Perhaps this means that every person can reach his full potential. After all, as the Chasidic master Reb Zusha said, we are held accountable only to the standard of our own potential - not more, but also not less.
The internalization of the fact that only Moshe Rabbeinu realized his full potential should itself be a humbling experience. On Rosh Chodesh Shevat, which represents potential, we should be moved to humility and a renewed effort to narrow the line which separates our actuality from our potential.