תניא רבן שמעון בן גמליאל אומר כל ששהה שלשים יום באדם אינו נפל שנאמר ופדויו מבן חדש תפדה שמנת ימים בבהמה אינו נפל שנאמר ומיום השמיני והלאה ירצה לקרבן וגו' הא לא שהה ספיקא הוי.
We learned in a Baraisa, Rabban Shimon ben Gamliel says, any human [newborn] that survives thirty days is not a stillborn, as it says (Bamidbar 18): And their redeemed, from the age of one month you shall redeem; any animal [newborn] that survives eight days is not a stillborn, as it says (Vayikra 22): And from the eighth day and onwards it will be accepted as an offering — but if it [the human or animal, respectively] did not survive, it is questionable.
A simple reading of the Gemara might lead us to conclude that it is essential to wait for eight days after an animal is born before slaughtering — and, that if the animal was slaughtered within eight days of its birth it may not be eaten, for it is questionable as to whether it was viable or still born — and a stillborn animal is forbidden.
In fact, the law is that in a case in which we know that the animal’s mother bore its newborn calf or kid for the full term of its gestation (which is nine months for large animals such as cows and five months for small animals such as sheep), the newborn may be slaughtered on the day that it is born.1 It is only in cases in which we are not sure whether the animal was carried for its full term that it is forbidden, as a possible stillborn, until the eighth day after its birth.2
According to most authorities, in the case of a healthy calf or kid where the only question is whether it completed its term of gestation or not, the requirement to wait eight days is only a rabbinic stricture.3 Accordingly, under certain conditions, such as where a calf that was less than eight days old became mixed up with calves that were more than eight days old, there is room for leniency.4 Similarly, although there is a dispute as to whether to the eight days end at the beginning of the eighth day (i.e., the night it commences),5 no matter how much “real-time” has elapsed, or whether the eighth day must incluse a minimum of seven twenty-four periods, if the slaughter took place after the earlier time, after, the fact ( בדיעבד ) there is room for leniency.6
On the other hand, what if it is definitively known that the animal did not complete a full term of gestation? In that case, many authorities are of the opinion that surviving for eight days is not enough. Rather, the calf or kid must be left to grow to maturity before it may be slaughtered.7
שו"ע יורה דעה סימן ט"ו סעיף ב': בהמה שילדה אם ידוע שכלו לו חדשיו דהיינו ט' חדשים לגסה וה' לדקה מותר מיד ביום שנולד ולא חיישינן שמא נתרסקו (פי' נכתשו ונכתתו) איבריו מחבלי הלידה.
שם: ואם אין ידוע שכלו לו חדשיו אסור משום ספק נפל עד תחלת ליל שמיני.
עיין בפתחי תשובה שם סק"ב ובדרכי תשובה שם סקכ"ב.
למרות שבכהאי גוונא העגל הוא דבר חשוב (בעל חי) ודבר שיש לו מתירין (לאחר ח'), עיי"ש בדרכי תשובה סקכ"ד.
[Although the eight days of bris milah require waiting until daybreak, that is becasue bris mila is a mitzvah that must take place during the day. Since shechitah may also take place at night, its eight day period can end immediately after the nightfall that ends the seventh day.]
עיין בפתחי תשובה שם סק" ב'-ג' ובדרכי תשובה שם ס"ק כ"ו-כ"ז.
עיין בט"ז שם סק"ד, בנקודות הכסף שהשיג עליו, ובדרכי תשובה שם סקי"ח.