Bechoros Chapter 4, Mishnah 1. We continue to deal with the laws of the firstborn of a kosher animal - a cow, goat, or sheep - whose firstborn male offspring is given to a kohen. The firstborn is given to the kohen as a gift, one of the priestly gifts, since the kohanim act as representatives, and it is as if the gift is given to Hashem. Giving a gift must be done in a manner of greatness and honor, in a fitting and respectable way; and just as one does not give a gift without wrapping it, so too it is not fitting to give an animal as a gift when it still requires a great deal of care.
Hence the subject of our Mishnah: its first section discusses how young the firstborn given to the kohen can be. A firstborn that is too young is not a gift but a burden, since the kohen will be forced to act as a farmer and care for the lamb, kid, or calf. Therefore, the Mishnah establishes that the Israelite must raise it until it can take care of itself, thus reducing the burden on the kohen.
How long an Israelite is obligated to care for a firstborn:
The Mishnah asks: "Ad kamah Yisrael chayav lehitapel bebechor?" - how much time is the Israelite required to care for the rearing of the offspring before handing it over to the kohen?
"Babehemah dakah sheloshim yom" - for a sheep or a goat, after thirty days he gives it to the kohen.
"Ubagasah chamishim yom" - for a cow, after fifty days he gives it to the kohen.
In doing so, the Israelite takes the burden of raising the tender, newborn offspring off the kohen.
"Rabbi Yosi omer badakah sheloshah chodashim" - according to Rabbi Yosi, the goat and sheep must be left with the Israelite for three months, since their teeth have not yet fully developed and they need to nurse, and taking them from their mother too early would cause difficulties for everyone.
The source for the measurements - an asmachta from the verse:
The halachah is ruled in accordance with the Sages: thirty days for a small animal and fifty days for a large animal. These measurements are based on a verse by way of an asmachta, meaning it is merely a hint. The verse deals with Bikkurim, Pidyon Haben (redemption of the firstborn son), and giving the firstborn animal:
"Meleischa vedimacha lo te'acher" - not to delay the bringing of the Bikkurim.
"Bechor banecha titen li" - the giving of the firstborn son in Pidyon Haben.
"Ken ta'aseh leshorcha letzonecha" - and so shall you do with the firstborn of the cows and the flock, meaning the goats and sheep.
Although the simple meaning of the verse does not deal with time measurements, Chazal derived them from the juxtaposition of the subjects within it:
Ox: The firstborn of the cow is juxtaposed with Bikkurim. The fruits begin to ripen near Pesach and are brought as Bikkurim on Shavuos, fifty days later - and from here we derive the measurement of fifty days for a large animal.
Flock: The firstborn of the goat and sheep is juxtaposed with Pidyon Haben. The son needs to be thirty days old, and on the thirty-first day he is redeemed - and from here we derive the measurement of thirty days for a small animal.
This is the source upon which the timeframes are based, and this is how the halachah is decided in practice by the Rambam.
When the kohen asks to receive the bechor within the time:
What is the law when the kohen asks to receive the bechor before its time? "Amar lo hakohen betoch zeman zeh: Tenehu li - harei zeh lo yiten lo" - If the kohen said to him within this time, "Give it to me," he should not give it to him. If on the twenty-seventh day, for example, the kohen asks to receive the bechor into his hands, the Yisrael does not give it to him. The reason is that it looks as though the Yisrael is casting his work upon the kohen, similar to someone who says to a kohen, "Help me with the threshing and I will give you the Terumah," which is forbidden because it looks like he is casting labor upon him. The gift must be given complete, without casting the work of caring for the animal onto the kohen.
However, in two cases it is permitted to give it even within the time:
A blemished animal: If the bechor is blemished, and the kohen said, "Ten li she'ochelenu" - "Give it to me so that I may eat it" - meaning he does not intend to care for it at all but rather to eat it immediately, it is permitted to give it to him, since it does not look at all as if he is doing the work of the Yisrael.
An unblemished animal during the time of the Temple: "Uvish'as hamikdash, im hayah tamim, amar lo: Ten li she'akrivenu - mutar" - And during the time of the Temple, if it was unblemished, and he said to him, "Give it to me so that I may offer it," it is permitted. Once eight days have passed, the bechor is fit to be offered, and therefore if the kohen says that he is going up to Jerusalem and asks to offer it and eat it immediately, it is permitted, as this does not look like casting work upon him, since he is asking for it for the purpose of immediate offering and eating.
"Bechor ne'echal shanah beshanah":
The final part of the Mishnah deals with the time for eating: "Bechor ne'echal shanah beshanah, bein tamim bein ba'al mum, shene'emar: Lifnei Hashem Elokecha tochalenu shanah bashanah" - A bechor is eaten year by year, whether unblemished or blemished, as it is stated, "Before Hashem your God you shall eat it year by year." The bechor is eaten within its first year, whether it is unblemished or blemished.
The Gemara explains that the year is measured according to the Hebrew calendar, from date to date - for example, from the seventeenth of Sivan until the following seventeenth of Sivan - and it makes no difference if it is a leap year, and the intention is not that everything must take place within a single calendar year.
It is worth noting that for an unblemished animal offered as a sacrifice, the first seven days are not counted, since it is not fit to be offered during them. The year is therefore counted from the eighth day onward, and this is the year during which it can be offered.
It turns out that a blemished animal must be eaten and not delayed, and an unblemished one must be offered and not delayed, and all within its first year. One who delays it beyond this nullifies this positive commandment.
In addition, as an independent point with many laws that this is not the place to expand upon, there is a general prohibition regarding holy items called "bal te'acher" (do not delay). Once a person is obligated in a sacrifice that he volunteered to bring, such as an olah or shelamim, he must bring it without delay, and the definition of a delay is when three festivals have passed and he has not brought it. A bechor is born already holy, and naturally the laws of "bal te'acher" apply to it, requiring it to be brought by the third festival, and one who delays it beyond the first year transgresses this prohibition as well.
In conclusion, one must remember that the common situation in our days is that the Temple does not exist. A blemished bechor is permitted for slaughter and eating, and the kohen eats it. But an unblemished bechor, which cannot be eaten due to its unblemished status and for which there is no Temple in which to offer it, will necessarily remain beyond its first year, and there is no avoiding this: we do not eat it early simply because the year is about to end.