One of most famous mitzvot of the Torah is that of shiluach haken, the obligation to send away the mother bird before taking her little chicks or even unhatched eggs. So important is this mitzvah that it is one of the very few in which we are promised long life for its observance.
This well-know mitzvah has a “cousin”, one not quite as well-known—that of oto v’et beno, the prohibition to slaughter a mother and its offspring, cow and calf, on the same day.
The Ramban (Devarim 22:6) explicitly links these two mitzvot, noting that both are meant to distance us from being mean-spirited and to inculcate within us the attribute of mercy. While the Torah allowed man—under strict conditions—to kill animals, to kill a mother and its offspring on the same day is just plain cruel. Interestingly, the prohibition of oto v’et beno does not apply to birds, and the mitzvah of shiluach haken does not apply to animals. These two mitzvot complement each other, stressing a similar idea regarding different species.
The Ramban adds a second possible reason for these two mitzvot: namely, that the killing two generations in one day is symbolically akin to the destruction of a species.
The link between kindness to animals and creation is reflected in the Mishna that teaches, “Regarding the ‘one day’ referred to in oto v’et bno, the day follows the night”. If one slaughters a cow on Tuesday night and the calf during the day on Wednesday (or vice versa), one would be in violation of this command. However, if the first slaughter took place on Wednesday during the day and the second on Wednesday night, the prohibition would not be violated.
This should be rather obvious, and the Gemara immediately wonders why there is a need for this teaching. Rabbi Shimon ben Zoma explains that, since the prohibition of oto v’et beno is written in the context of the laws of korbanot (Vayikra 22:28), one might have thought that this law is patterned after that of sacrifices, where the rule is that the “night follows the day”, i.e., the day starts at daybreak and ends the following morning. Thus, if one brings a thanksgiving offering on Tuesday afternoon one must, according to biblical law, finish eating it before daybreak on Wednesday. The rabbis, worried that one might mistakenly eat after dawn, insisted that we finish before “midnight”, i.e., the midpoint of the night. It is for this reason that the Afikoman is to eaten before midnight. Yet, despite its placement in the Torah, the day of oto v’et beno is that of a “regular” day beginning at night.
The Mishna derives this law by noting the similar language, that of “yom echad,” used both regarding oto v’et beno and the creation of the world, illustrating that violation of this command is an attack on creation itself.
It is this second approach to killing two generations on the same day that explains the logic of this prohibition being violated, even unwittingly. Thus, if Reuven slaughters a cow in the morning and Shimon slaughters the calf in the afternoon, the prohibition would be violated, albeit unintentionally and unknowingly.
With the chances of cow and calf being slaughtered on the same time generally very low, we need not concern ourselves with such an eventuality—except on special occasions.
“Four times a year, one who sells an animal to his friend must inform him ‘I sold the mother to be slaughtered, I sold the daughter to be slaughtered’” (Chulin 83a).
With an obligation to eat meat on Yom Tov, demand for animals was most high on erev Yom Tov. Absent refrigeration, the only way to get tasty—and perhaps healthy—meat for Yom Tov was to have the animal slaughtered just hours before one’s meal. As one could not freeze or even refrigerate meat, slaughtering animals was permitted on Yom Tov itself. This all meant there was a real chance that mother and calf would be slaughtered on the same day. Hence, the need to let customers know that they must be careful not to violate this prohibition.
These four times are “the eve of the last day of Sukkot, the eve of the first day of Pesach, the eve of Shavuot and the eve of Rosh Hashanah. And according to the words of Rabbi Yossi HaGilili, the eve of Yom Kippur in the Galilee as well”.
Tosafot (s.v. ukdivrei) notes that missing from this list is erev Sukkot. In a fascinating comment, the Tosafists note that, “the whole world is busy with [the] sukkah and lulav and they do not have so much free time to be involved in shechita.” Hence, we need not fear that mother and calf will be slaughtered on the same day. In other words, the average person was busier on erev Sukkot than on erev Pesach. While this might be because erev Pesach is just too late to prepare for Pesach, it is more likely due to the fact that the level of cleaning done today far, far exceeds the halachic requirements of ridding our homes of chametz. Many thus dread the arrival of Pesach, thereby nullifying the mitzvah of simcha, joy, that is so integral to the celebration of Yom Tov.
 The Ramban explicitly rejects the Talmudic view (Brachot 33b) that “one who says Your mercy extends to the birds in the nest, we shut him up”, arguing that this view reflects the rejected rabbinic notion that mitzvot lack human rationale and are but divine orders to be obeyed, period—a view that in modern times was articulated by Yishayahu Leibowitz.
 It is the differing observations of the days that make the seder night so special. Chag haPesach begins with the bringing of korban pesach in the Temple on the 14th of Nissan and thus, ends the next morning. Chag haMatzot, the seven-day festival having little to do with the Temple, begins on the night of the 15th of Nissan (in regular Jewish time), making the night of the 15th the conjugation of these two nation-forming holidays. No wonder the pesach and matzah are to be eaten together.
 Rav Shimshon Raphael Hirsch beautifully explains that the Temple, the resting place of the Divine presence, is the source of light for the world. Hence, the day begins and ends with light. As we distance ourselves from G-d’s presence, we are enveloped in darkness and therefore, outside of the Temple, the day begins and ends with darkness. (I recall reading this many years ago, but have been unable to locate the source. I would be most grateful if someone might enlighten me.)
 The term hamocher lechaveiro, selling to a friend, does not mean literally mean a friend, and the same laws apply even when selling to a total stranger. Rather, the use of the term chaveiro is indicative that we are to treat all we come in contact with as our friends. We are “chaverim kol yisrael”, a message especially important as we celebrate our holidays.
 Apparently, Jews in other locales preferred chicken before the fast. Whether this is at all connected to the view of Rav Yossi Hagilili that one may cook, eat and otherwise enjoy chicken and milk together, I can only speculate. Perhaps chicken was so rarely eaten in the Galilee that there was little need to make a decree forbidding eating chicken and milk together out of fear that one might mix up chicken with meat.
 Something that no doubt pleases those who run Pesach programs, an industry that likely generates over half a billion dollars in revenue over the course of the holiday.