It is rare to find a substance so utterly proscribed by the Torah. There are other foods whose consumption is forbidden; but this the Torah forbids us to eat, benefit from in any way, or even keep in our possession. Usually, a forbidden substance becomes “nullified” if it mixes with a much greater quantity of permissible substances; of this, the Torah forbids the slightest trace—even if it blends with something a million times its volume, the entire lot becomes unfit for consumption.
We are speaking, of course, of chametz, or leaven, on Passover. In the weeks before the festival, the Jewish home is the scene of an all-out war of extermination. Floorboards are scraped, furniture dismantled, countertops boiled. On the night before the festival we conduct a solemn search for any survivors and consign them to the flames on the next morning. The enemy: the most minute bread-crumb, beer stain or pasta residue—anything in which grain and water have come together and fermented, rendering the product chametz and utterly intolerable for eight days a year.
On the spiritual level, leaven, whose primary feature is that it rises and inflates itself, embodies pride. This explains our uncompromising rejection of chametz. Other negative traits might be tolerable, or even useful, in small, greatly diluted doses. Depression, for example, has been declared “a grave sin,” for man is commanded to “serve G-d with joy”; but a small dash of melancholy, counterbalanced by a hundredfold helping of joy, may serve a positive function, reflecting a necessary concern over one’s shortcomings and the commitment to rectify them. The same applies to anger, stubbornness, chutzpa, and a host of other negative character traits: as a rule, they are undesirable, yet in the proper context and in the right proportions, each has its positive applications. Arrogance and pride, however, are of such spiritual toxicity (the Talmud states that G-d says of the arrogant one, “I and he cannot dwell in the same world”) that we must forgo any attempt to exploit them, and must totally eradicate them from every crevice of our hearts.
The 49-Day Difference
And yet, despite the severity of the prohibition of chametz, it is only forbidden for eight days and several hours a year, while other, less “toxic” elements are forbidden year-round. In other words, there is a state of being, which Passover represents, in which arrogance and pride are objectionable in any context and quantity. After “Passover,” however, chametz becomes permissible and even desirable.
This duality is also expressed in the laws governing the offerings brought to G-d in the Holy Temple in Jerusalem. In the Holy Temple, it was “Passover” all year round: all grain offerings had to be unleavened, in keeping with the divine command that, “No leaven … may be brought as a fire-offering to G-d.” This, too, reflects G-d’s utter abhorrence of arrogance and pride. Nevertheless, on the festival of Shavuot, two loaves of bread, specifically commanded to be “baked leavened,” were offered in the Temple.
Thus, Passover and Shavuot represent two extremes in the desirability of pride. On Passover chametz is wholly and utterly forbidden, while on Shavuot it is not only permitted but is a mitzvah, commanded and desired by G-d.
Passover marks our birth as a people, when G-d extracted a clan of slaves from the “forty-nine gates of depravity” of Egypt and set them on the journey toward Sinai, where He took Israel as His eternal bride on Shavuot. Connecting Passover and Shavuot is the forty-nine day “Counting of the Omer”: the Torah commands that beginning on the eve of the second day of Passover, we should conduct a daily count of the days that have passed from the day after the Exodus.
The Kabbalists explain that the human personality consists of seven basic attributes (love, restraint, harmony, ambition, devotion, connection and receptiveness), reflecting the seven divine attributes (sefirot) which G-d invested in His creation. Each sefirah contains elements of all seven, making for a total of forty-nine divine channels of relation to our reality, and forty-nine corresponding traits in the human heart. Thus, the Kabbalists speak of the utterly corrupt society of Egypt as a moral nadir of “forty-nine gates of depravity.” These are paralleled by “forty-nine gates of understanding”—the ladder and process by which one achieves the refinement and perfection of all elements in one’s character.
Therein lies the significance of the forty-nine day count and climb from Passover to Shavuot. On the first day of Passover, we were physically removed from the land of Egypt; yet we still had to remove the “Egypt” from within us, to cleanse our hearts and minds of the residue of two centuries of pagan environment and practice. So on the second day of Passover begins a forty-nine-day count, chronicling a daily internal exodus from another of Egypt’s “gates of depravity” and entry into another of the “gates of understanding.” After forty-nine days, we attain the internal purity required to receive the divine election and communication of Shavuot.
Hence the difference between Passover and Shavuot regarding chametz. One who is still burdened with negative drives and emotions lacks the ability to sublimate the most potent and corruptible of the heart’s traits—pride. So immediately following the Exodus, chametz is banned. It is only upon attaining the full refinement of all forty-nine compartments of the heart on Shavuot that the offering of leaven to G-d becomes a mitzvah, appropriate and desirable.
On this level, pride is no longer the self-inflating chametz of the “Passover” personality, but the selfless pride of one who has cleansed his heart of every last vestige of self-interest and has dedicated it exclusively to the service of his Creator. This is a pride not in what one is or has achieved, but an expression of the majesty of He whom he serves and whose reality he conveys in his every thought, word and deed. 
This also explains an interesting law regarding Acharon Shel Pesach, the eighth and final day of Passover.
One example of the extremes to which we go to avoid the slightest trace and chance of chametz on Passover is the practice, in many communities, of refraining from eating matzah sheruyah (“soaked matzah”) on the festival.
Matzah is made of water and flour that have been speedily and thoroughly blended and baked, to avoid any chance of leavening. Once baked, the flour in the matzah will not leaven; matzah (or “matzah meal” made by grinding matzah to a fine “flour”) may now conceivably be mixed with water and other liquids in the preparation of food for the festival. However, there remains an extremely slight chance that some of the flour might have failed to mix completely with the water at the time of the matzah’s original baking, leaving a few particles of raw flour at risk of leavening should they come in contact with water. For this reason, many halachic authorities, including Rabbi Schneur Zalman of Liadi, rule that it is best to avoid the use of matzah sheruyah on Passover. This ruling has been accepted by many communities, to the extent that there are those who are careful not to even place matzah on the table during mealtimes unless it is securely covered, lest a single drop of liquid alight on a piece of matzah. This is one of the many examples of the unparalleled lengths to which we go in the avoidance of chametz on Passover.
On the other hand, Rabbi Schneur Zalman permits the use of matzah sheruyah on the eighth day of Passover. Furthermore, his successors, the Rebbes of Chabad, made a point of wetting matzah at every course of the meals of Acharon Shel Pesach.
There are those who are wont to explain this leniency by the fact that the eighth day of Passover is a rabbinical institution, as opposed to the first seven days, which are biblically ordained. But the observance of the rabbinical “added days” to the festivals are just as binding for the Jew as their biblical sisters; in fact, Halachah is even more stringent regarding certain aspects of their observance, for the very reason of forewarning any inclination to treat them lightly. Indeed, with the exception of the eating of “soaked matzah,” we are no less diligent in our rejection of leaven on Passover’s final day. Why, then, this exception?
A Taste of Future
As we have discussed, the forty-nine-day count from Passover to Shavuot represents the process of refining the seven basic attributes of the heart as each comprises elements of all seven, making for a total of forty-nine traits. This is why the Torah speaks of the count as consisting of weeks (“Seven weeks you shall count for yourselves…”). In our daily count, we, too, emphasize its weeks: on the twenty-fifth day, for example, we say, “Today is twenty-five days, which are three weeks and four days to the Omer [count].” Indeed, Shavuot—the name of the festival that culminates the Counting of the Omer—means “weeks.” For the internal “count” also consists of seven “weeks,” being the refinement of seven attributes of the heart that are each a unit of seven.
Thus, each week of the count is a microcosmic Omer Count of its own, involving seven “days” or sub-traits as they are reflected in the various nuances of that week’s “attribute.”
The eighth day of Passover is the seventh day of the count and the final day of its first week. It therefore represents the point at which elements of each of the seven attributes (as they are present within the attribute of “love”) have been refined and elevated. The eighth of Passover is thus a mini-Shavuot, and shares its leaven-tolerant quality. While outright chametz is still strictly forbidden, we mark this milestone on the road to perfection with the positive use of a chametz-vulnerable element, employing wetted matzah to enhance our festival meal.
This corresponds to another feature of the eighth day of Passover—its identification with the era of Moshiach. The haftorah (reading from the Prophets) for this day (Isaiah 10:32-12:6) describes the coming of Moshiach and the harmonious perfection of a time when “the world shall be filled with the knowledge of G-d as the waters cover the sea.” Rabbi Israel Baal Shem Tov instituted a special meal, “The Feast of Moshiach,” to be held on the afternoon of eighth day of Passover, as a time that is profoundly suited to “taste” and experience the divinely perfect world we are creating with our positive efforts—a world in which “the spirit of impurity shall cease from the earth” and everything, including the pride so abhorrent to G-d today, shall be sublimated as a wholly positive and altruistic force.
Therein lies the lesson of the eighth day of Passover: even if perfection seems a far-off goal, you possess the ability to create a “taste” of perfection in the here and now. Start with a single trait of your personality, with a small corner of your community. If you wholly devote yourself to it, you will find in it elements of your entire self, indeed of the entire universe. Your creation of this small model of perfection will serve as the catalyst for its realization on a holistic, and ultimately universal, level.
Based on the Rebbe’s talks on the eighth day of Passover in the years 5727 (1967) and 5737 (1977) and on other occasions 
Adapted from the teachings of the Rebbe by Yanki Tauber
. Certain forbidden substances are nullified if they constitute a minority of the mixture; others if they are less than one-sixtieth, less than one part in one hundred, or less than one part in two hundred. It should be noted that it is forbidden to intentionally nullify a prohibited substance.
. From a saying by Rabbi Israel Baal Shem Tov (Keter Shem Tov, section 169).
. Psalms 100:2.; see Tanya, chapter 26.
. Talmud, Erchin 15b.
. Cf. Maimonides’ commentary on the Mishnah, Ethics of the Fathers 4:4.
. From mid-morning of the day before Passover, until nightfall of the eighth day (seventh day in the Land of Israel).
. Leviticus 2:11.
. Ibid. 23:17.
. E.g., loving in love, restraint in love, harmony in love, etc.
. Cf. II Chronicles 17:6: “And his heart was lifted up in the ways of G-d.”
. Responsa 6, printed at the end of Shulchan Aruch HaRav.
. See Tur, Shulchan Aruch and commentaries, Orach Chaim, section 491.
. Deuteronomy 16:9; see also Leviticus 23:15.
. Deuteronomy, ibid., v. 10.
. Zechariah 13:2.
. The “Feast of Moshiach” offers another, even more extreme example of Acharon Shel Pesach’s “tolerance” for chametz. On Passover (as on all other festivals) a special section—Ya’aleh Veyavoh—is added to the Grace After Meals recited at the conclusion of each meal; this section includes the passage, “Remember us for good, on this day of Passover.” The law is that if one begins his meal on the last day of a festival and continues it after nightfall, he is to recite the Ya’aleh Veyavoh at the conclusion of this meal, even though, for everyone else, the festival has ended many hours ago. Nevertheless, it is permissible to eat chametz immediately after nightfall of the last day of Passover, even in the middle of a meal that began before nightfall. Thus we have the amazing paradox of a Passover meal, at whose conclusion we still consider ourselves “on this day of Passover,” during which it is permissible to eat chametz! (Shulchan Aruch HaRav, Orach Chaim, 491:3. See also Mishnah Brurah, ibid.; Likkutei Sichot, vol. XXII p. 36, notes 62-64).
. Likkutei Sichot, vol. XXII, pp. 30-38.