1) “During the entire month of Nissan one does not say Tachanun…” (Kitzur Shulchan Aruch 107:1)
The Mekor Chaim, zt”l, explains that Tachanun is usually said in order to bring about the downfall of the sitra achra, the “side” of spiritual impurity. We cast ourselves down to symbolize the descent into the sitra achra, and by the time we come up we have extracted holy sparks from the abyss. By doing this, the hidden light of holiness which has been swallowed by the other side reverts to the side of kedushah.
However, during Nissan—the time of the great miracles of the birth of the Jewish people—the sitra achra has already been subdued and there is no need to do this rectification. On the contrary, not only does the sitra achra have less power, but a little teshuvah during this holy month goes a very long way!
The Chesed L’Avraham, zt”l, writes that between Purim and Pesach we escape the forty-nine levels of impurity a little at a time. Each day, we are removed from a bit more defilement. By the time Rosh Chodesh Nissan arrives, we are sufficiently removed that we don't even need to say Tachanun to be worthy of a true connection with Hashem. As we have already seen, Tachanun clears away our blemishes. The Arizal explains that although our Shemona Esrei is a connection to Hashem, it is incomplete without the Tachanun prayer. It is Tachanun that removes the sins swallowed by the other side that block our ability to really connect with Hashem. First, we do teshuvah. Then we “fall” in order to show that we are rectifying the damage done by our sins. Only after this process is our connection to Hashem through our Shemona Esrei truly consummated, since the sins impeding us have been removed.
During the month of Nissan, we are already elevated to such an extent that we do not need this process to remove the residue from our sins which would otherwise stand in our way. At this time, our teshuvah and dveikus during the Shemona Esrei alone is enough to merit true connection. This is because Hashem has sufficiently removed us from our personal forty-nine levels of defilement that keep us from living the fulfilled and joyous lives that we all instinctively know we should be living. Our ego gets in our way with its harmful pride and we cannot enjoy our blessings. So we go down in Tachanun to represent the fact that we are nothing at all and that nothing is coming to us. We then remove the sparks which the other side grabbed hold of because of our tremendous arrogance. This is why in earlier times people would literally lie prostrate on the ground during the prayer—complete prostration is the embodiment of humility. (Based on Likutei Halachos).
2) “We do not fast during the month of Nissan, even for a Yahrtzeit…” (Ibid., 107:2)
Rav Pinchas of Koretz, zt”l, explains that everything in the natural world comprises an aspect of katnus (“smallness” / “immaturity”) and gadlus (“greatness” / “maturity”). The katnus of the thing is always in inverse proportion with its real significance in the scheme of things. For example, although a day-old calf can already walk, although its katnus state is quite advanced, its gadlus state is spiritually undeveloped. In contrast, a day-old infant can do nothing and must be well swaddled and protected to ensure its survival. Even if it takes a baby as long as two years to master walking, this is still within the range of normal. There is no creature with as undeveloped katnus as a human being. The reason for this is because once a person comes to gadlus, he can come to great levels of gadlus. Mankind rules over all of creation. If he is worthy, a person can even rule over angels!
Sleep is also an aspect of katnus and so there is also a difference between creatures in this area. A horse, for example, sleeps standing up. Most beheimos tehoros sleep on their knees (at least part of the time). A human being, however, lies down to sleep—the most vulnerable and “undeveloped” position. The katnus of the sleep state is in an inverse relationship with the gadlus one attains while awake and mentally active. As the verse says, “chadashim la-b’karim”—“one is renewed each morning.” A person is renewed each day with increased understanding and more maturity. An animal’s mentality is negligible compared with that of a person; their gadlus is limited, so the katnus of their sleep state is far less and they need not lie down.
Another example of the katnus that precedes gadlus is this seemingly endless exile that we are enduring. When Moshiach comes we will merit intensely strong gadlus, and so we must first endure overwhelming katnus at great length. Fasting is also an aspect of katnus as we find in Pri Etz Chaim. For this reason, a bride and groom fast on the day of their wedding, and this is why it is very important to fast in general. Without experiencing the requisite degree of mochin d’katnus, one is unable to receive the mochin d’gadlus that are in store. This is one reason why there are a minimum number of fasts throughout the year. Rav Pinchas of Koretz even recounted that he had known several cases of people who were chronically ill because they had blemished their mochin d’katnus. He told them to fast and they subsequently enjoyed quick recoveries.
During Nissan, we are gifted with an abundance of holiness from on high. This is the wrong time to be fasting to correct the state of katnus. During Nissan, we are in an aspect of gadlus and fasting would only blemish this state. This is true of all fasts except for that of the firstborn on Erev Pesach (see Spirit of the Law—Pesach 113) and the fast of a bride and groom. This latter fast is a very important means of preparing for the couple’s new life together. Without marriage, one is a broken half. We need a fully rectified state of mochin d’katnus to be able to receive this intense gadlus for life called marriage. Even though we are in an aspect of gadlus in Nissan, our level before marriage compared with our level after marriage is like entering the greatest gadlus from the greatest katnus. Therefore, a bride and groom should fast on the day of their wedding.
3) “During Minchah of Shabbos Hagadol, one should recite the Haggadah instead of Barchi Nafshi since this Shabbos was the beginning of the miracles and the salvation…” (Ibid., 170:3)
Rav Nosson, zt”l, writes that all miracles come in the merit of Shabbos. This is because all miracles are a result of Hashem’s special providence over the whole world which is an aspect of the world to come. Shabbos is a mini-taste of the world to come in this world. Therefore, all miracles come from Shabbos, and this is why the miracles and the salvation of Pesach started on Shabbos.
This Shabbos is called Shabbos Hagadol since miracles are referred to as “gadol”—“great things.” (We see this in the verse: “Recount to me the great things that Elisha wrought.” [Melachim II:8:4]) All of these miracles were achieved in the merit of the special providence that is drawn into the world on Shabbos.