Parah, Chapter 1, Mishnah 4. The opening chapter of this tractate is occupied with the ages of animals brought as korbanos: the years that define a lamb and a ram, and, in this mishnah, the earliest age at which an animal may be brought at all. Some korbanos require an animal that has reached thirty days, and others may be brought as soon as the animal has completed seven days of life.
The mishnah begins with the korbanos that call for the older age. The list includes the "chatos hatzibur" along with "oloseihen", the communal chatas offerings together with the olos that accompany them; the "chatas hayachid", a chatas brought by a private individual; and two ashamos, that of a "nazir" and that of a "metzora". Of all of these the mishnah rules "kesheirin miyom sheloshim vahalah", valid once day thirty has arrived, adding "ve'af beyom sheloshim", that the thirtieth day itself is included. A complete month of life need not have gone by; the moment the animal reaches its thirtieth day it already qualifies.
A further clause follows: "Ve'im hikrivum beyom shemini, kesheirim." Should someone have gone ahead and brought one of these korbanos when the animal had only completed its seven days, the korban stands. Thirty days is what this category calls for, yet an animal brought earlier is not thereby rendered pasul.
Now the second category: "Nedarim unedavos, habechor, vehama'aser, vehapesach", animals designated through vows or free-will gifts, a bechor, ma'aser behemah, and the korban Pesach. Concerning these the mishnah teaches "kesheirim miyom hashemini vahalah", their eligibility beginning once seven days of life have passed, and it stresses "ve'af beyom hashemini", that the eighth day itself already serves. As in the earlier case, the animal may be brought on the very day it attains the minimum age.
It is worth noticing the contrast with the animal of thirteen months, which is caught between being a lamb and being a ram and is fit as neither. Here there is no such in-between day. The thirtieth day is itself a valid thirtieth day, and the eighth day is itself a valid eighth day; the animal is a kosher korban on the day it arrives at the required age.
One point calls for explanation: the Pesach is an obligatory korban, not a gift of the heart, so why should it be permitted already at seven days completed? One answer offered is that the demand for these lambs was overwhelming, since every group of participants in Yerushalayim needed a lamb of its own; had the law insisted on thirty days, the supply might well have fallen short. So writes the Maharam, and the Bartenura points in much the same direction.