Commentary for Numbers 18:18

כחזה התנופה וכשוק הימין [AND THE FLESH OF THEM (of the firstborn animals) SHALL BE TO YOU] AS THE WAVE BREAST AND THE RIGHT SHOULDER of the festival offerings which may be eaten by the priests, their wives and their sons and men-servants during a period of two days and one (the intervening) night, reckoning from the day of slaughtering; so, too, the flesh of the firstborn animal sacrifice may be eaten during two days and one night (Sifrei Bamidbar 118; Zevachim 57a).

The firstborn also… Meaning: The Torah compares the firstborn to the breast and the thigh of a peace-offering, they are also of lesser holiness and are eaten for two days and one night.

V. 18. ובשרם יהיה לך sowohl תם nach זריקת הדם und הקטרת החלב des vorigen Verses, als auch בעל מום, der nicht zum קרבן tauglich ist, gleichwohl dem zum Eigentum und Genuss wird und ihm nur אסור בגיזה ועבודה bleibt (siehe Dewarim 15, 19 f.; Sebachim 37a). — כחזה התנופה וכשוק התרומה לך יהיה: wie חזה ושוק von שלמים, die נאכלין לשני ימים ולילה אחד (siehe Dewarim 7, 16 und Sebachim 57a) und die oben V. 11 allen Gliedern des כהן-Hauses zum Genuss überwiesen sind (Bechorot 32a).

ובשרם יהיה לך, “however, their flesh is yours.” This even includes a blemished firstborn, which is donated to the priest, something we do not find elsewhere in the Torah. (Sifri)

לך יהיה IT SHALL BE THINE — Rabbi Akiba came and taught: By repeating the words לך יהיה Scripture adds another “status” (more lit., state of being; i.e., it intends the status implied in the first לך יהיה to be extended), in order that you should not say that it shall be to them even as the breast and shoulder of the thanksgiving offering, which may only be eaten during one day and the following night (Sifrei Bamidbar 118; Zevachim 57a).

R’ Akiva came. Re’m writes: One may wonder why one needs the comparison? Let us learn it from the verse, “Year by year” (Devarim 15:20). There Rashi explains, “If one slaughtered it at the end of its first year then one could eat it on that day and for one day into the next year; this teaches that the firstborn is eaten for two days and one night.” It appears to me that if not for the words “it shall be yours” one would have thought that “year by year” meant the opposite, that if one slaughtered it at the end of the year it would only be eaten on that day and not in the next year. Therefore, the Torah writes, “It shall be yours” to add an additional “Be…” Inevitably we then expound that “year by year” means that “If [one slaughtered] it at the end of its first year…”