The Gemara teaches in Masechet Ta'anit (29), "Mi'shenichnas Adar Marbim Be'simha" – "When Adar comes, we increase our joy." Rashi explains, "Because of the miracles that were performed for Israel – Purim and Pesah." According to Rashi, then, the special joy of Adar is due to two miracles – the Purim miracle, and the miracle of the Exodus which we celebrate on Pesah. It thus emerges that the obligation to be joyful applies not only during Adar, but also during Nissan, in which we celebrate the miracle of Yesi'at Misrayim. Our Rabbis explain this observance on the basis of the concept of "Hazaka." When something happens once, it could be just a fluke, and does not necessarily reflect a consistent pattern. But when it happens twice (according to one view in the Gemara), then this establishes a "Hazaka," a pattern. And thus if the Jewish people experienced great miracles on just one month, this would not necessarily prove that we are blessed with good Mazal (fortune). But once we have two consecutive months of great miracles, we have a "Hazaka" of favorable Mazal which warrants joy and celebration. Besides the miracles of Purim and Pesah, there is also an additional reason for the special joy of Adar. Rav Levi Yishak of Berditchev (Ukraine, 1740-1809), in his Kedushat Levi (Parashat Mishpatim), notes that the month of Adar always begins immediately following the period of "Shobabim," the six weeks when we repent and seek to correct sins associated with the Berit. During this period, we attempt to retrieve the sparks of holiness that had been seized by the Sitra Ahara, by the forces of impurity associated with Esav. The Torah describes Esav at his birth as "Ke'aderet Se'ar" ("as though dressed in a cloak of hair" – Bereshit 25:25). And thus when the Sages teach, "Mi'shenichnas Adar Marbim Be'simha," it means that when we retrieve the sparks from "Adar" – from Esav, who is referred to as the "Aderet" – we are joyful. We celebrate in Adar because we have restored the sparks of holiness through our repentance during "Shobabim," and we are thus able to serve Hashem with greater purity. Indeed, the month of Adar is associated with Yosef Ha'sadik, who embodies the ideal of Shemirat Ha'yesod, preserving the sanctity of the Berit. It is therefore written that one should be especially careful during the month of Adar to avoid wine (except on Purim day, of course), which could lead to inappropriate conduct, and to ensure that he does not stumble upon sin, Heaven forbid. The constellation of the month of Adar is the fish (Pisces). The word "Dagim" ("fish") represents the phrase "Yod Gimal Mechilan De'rahameh" – the thirteen attributes of compassion. The month of Adar is a special month of mercy and kindness for the Jewish people, when we are able to receive direct influence from the highest levels in the heavens. This opportunity – to be at the level of Keter and receive blessings from the highest levels of sanctity – is normally reserved for Minha time on Shabbat, but we also have access to this power during the month of Adar, and especially on Purim day itself. This is a month that is especially well-suited for prayers, for striving for holiness, and for receiving Hashem's great blessings. For this reason, our Sages teach that if one has a court case against a gentile, he should try to schedule the trial for the month of Adar, as this is a special month of joy, blessing and good fortune.