Vayikra 22:27 – An animal that is sacrificed must be at least eight days old

ויקרא כז:כב - שור או כשב או עז כי יולד והיה שבעת ימים תחת אמו ומיום השמיני והלאה ירצה לקרבן אשה לה'.

Vayikra 22:27 records that an animal must be at least eight days old to be acceptable as a sacrifice, as for the first week it should be with its mother. Similarly, Shemot 22:29 records that the firstborn animal can only be offered as a sacrifice when it is eight days old. Why does an animal have to be at least eight days old to be a sacrifice?

Milgrom (2000, p. 1883) writes that the most popular rationale is that the law is based on humanitarian concerns not to be cruel to the animal. However, he rejects this since the animal can be offered as a sacrifice on the eight day. Why is killing the animal on the eighth day more humane than on the seventh day?

I prefer R. Shimon ben Gamliel’s approach (Shabbat 135b, quoted by the Bekhor Shor on 22:27) that if the animal lives eight days then it is not considered an aborted fetus. Why should it matter whether the animal is considered an aborted fetus or not? The Hinukh (positive mitzvah 267, also quoted by N. Leibowitz, 1980, pp. 206,207) writes that prior to eight days the animal has no value. The Abravanel (2005, p. 239) writes that prior to the eighth day, the animal is considered as blemished.

I would slightly vary this idea. During the animal's first week there is a higher probability that that it will not live, as opposed to after the first week. Even if the animal appears to be healthy from the outside, maybe there are some internal problems that will cause it to die. Accordingly, if a person is not sure whether the animal is viable maybe he is offering it as a sacrifice since he thinks it will die anyway. This is an unacceptable sacrifice, just like when a person offers a blemished animal as a sacrifice since then possibly the animal is being offered to remove a “defective animal.” However, once the animal has lived a full week, then the animal is considered viable, and the person is not trying to offer a “free” sacrifice.