Tafsiran Keluaran 22:29: Rashi, Sforno, Siftei Chakhamim & Rav Hirsch

שבעת ימים יהיה עם אמו SEVEN DAYS IT SHALL BE WITH ITS DAM — This is an admonition for the priest (for the Israelite himself does not sacrifice the firstborn animal) that if he wishes to bring his offering of this at an earlier date (i.e. in the case of the Israelite having given it to him before the thirtieth or fiftieth day) he is not allowed to offer it before it is eight days old because it is then “short in time” (i. e. too young for sacrifice; cf. Leviticus 22:27).

כן תעשה לשורך לצאנך, that you shall give to Me the firstborn which is not a prematurely aborted fetus. Our sages have said (Shabbat 135) that “a human baby which remains alive for 30 days is considered viable, not in the category of נפל, prematurely aborted. In the case of animals the parallel period is eight days.”

This is a warning to the kohein. However, a person may consecrate his animal [as a sacrifice] even if it is only one day old. This warning, [i.e.,] negative commandment, is part of the positive commandment of “Seven days it shall remain. . .” [and thus is judged as a positive commandment].

ביום השמיני תתנו לי ON THE EIGHTH DAY THOU SHALT GIVE IT ME — One might think that it is obligatory to offer it on that day! Scripture, however, uses here the words “eighth day” and uses them again later on, (Leviticus 22:27) “and from the eighth day and henceforth it shall be favourably accepted”, in order to suggest an analogy. Now what is the meaning of the “eighth day” mentioned in the latter passage? It intends to declare the first-born fit for sacrifice from the eighth day and henceforth (i. e. it is intended to fix the earliest possible time on which it may be sacrificed). This, too, in the purpose of the word שמיני used here: to declare it fit for sacrifice from the eighth day and henceforth. The text therefore implies: “on the eighth day thou mayest give it to Me” (and not “thou shalt give it to Me”) (Mekhilta).