Chullin, Chapter 5, Mishnah 5. This chapter has been occupied with the prohibition of oto ve'et beno, slaughtering a mother animal and its offspring on the same day. Until now the Mishnah has clarified who is liable, in what circumstances, and how the prohibition applies. Now it turns to a more basic question that everything else depends on: when exactly does that "day" begin and end?
The Torah forbids slaughtering the mother and the young "on one day." But the word "day" is not self-explanatory. In most areas of halacha, a day runs from nightfall to the following nightfall: first the night, and then the daytime that follows it. When it comes to korbanot, however, the halachic day works differently. There the day begins at daybreak and continues until dawn of the next morning, so that the night follows its day rather than preceding it.
Why would anyone think korbanot are the relevant model here? Because of context. The prohibition of oto ve'et beno appears in the Torah immediately after an extended discussion of korbanot. A reader could reasonably conclude that the "day" of oto ve'et beno should be measured the same way as the day of the sacrifices, from daybreak to daybreak. Our Mishnah comes to reject that assumption.
The Ruling
Here is the ruling. Concerning the "one day" that the Torah specifies in the law of a mother and her young, the Mishnah lays down the principle "hayom holech achar halaylah", meaning that daytime trails after the night that came before it. The halachic unit of twenty-four hours therefore opens when night falls: night first, and the hours of daylight attached to it afterward.
The practical outcome is straightforward. Once one of the two animals has been slaughtered, the second one, whether the mother or the offspring, may not be slaughtered until nightfall arrives. At nightfall the legal day turns over, a new day begins, and the second animal may then be slaughtered.
The Proof from Creation
The Proof from Creation
The Mishnah does not let the ruling stand on its own authority. It supplies a source from the pesukim and credits the derashah to a Tanna, Rabbi Shimon ben Zoma, introducing it with the words "et zo darash", this was the exposition he offered.
"Ne'emar bema'aseh bereishit": a certain expression is used in the Torah's account of Creation, namely "yom echad", one day. The verse runs "vayehi erev vayehi voker", and there was evening and there was morning, and it closes with those very two words, "yom echad". "Vene'emar be'oto ve'et beno": and that identical pair of words is what the Torah employs in the verse forbidding the slaughter of a mother together with her offspring.
Because one expression serves in both passages, each can shed light on the other. Ask "ma yom echad", what sort of "one day" is intended where the phrase appears in the Creation narrative, "ha'amur bema'aseh bereishit"? There the darkness comes first and the light comes afterward, which is exactly the seder the verse itself spells out: evening, and only then morning. "Af yom echad", so likewise the "one day" of our prohibition: its daylight hours are pulled along after the night that began them, "hayom holech achar halaylah".
The lesson, then, is carried by the words "yom echad" themselves. Despite the proximity to the laws of korbanot, the day of oto ve'et beno is the ordinary halachic day: it opens at nightfall and closes at the following nightfall.