A first-born animal – a behor – is considered to be holy to the Temple (see Sh’mot 13:12). In the event that the animal develops a permanent blemish – a mum – then it is no longer kodesh and it can be eaten normally by kohanim.
Since under normal circumstances a behor cannot be eaten, it is not considered an animal that is ready for use on Yom Tov. Nevertheless, the Mishna (25b) teaches that according to Rabbi Yehuda, in the event that a behor falls into a cistern, an expert can be lowered into the cistern to check whether the animal has developed a mum. If, in fact, such a mum is present, then the animal can be slaughtered and eaten by kohanim on Yom Tov. Rabbi Shimon disagrees. He believes that unless the mum was recognized before Yom Tov began, the animal cannot be used for food on Yom Tov. Thus there would be no point in having an expert check the animal for a mum on Yom Tov itself so it would be forbidden to do so.
- Ruling that the animal can be eaten is tikkun – it is “fixing” something on Yom Tov – which is forbidden.
- Ruling that the animal can be eaten is considered a formal court ruling. Jewish courts neither sit nor rule on cases on Shabbat or Yom Tov.
Rabbi Yehuda would reject both of those assumptions.
Tosafot offer another approach. According to Tosafot the main issue in this case is muktze. Since the behor was perfectly healthy at the moment that the holiday began, it was not viewed as edible at that point. Therefore it cannot be considered prepared for use on Yom Tov, even if it develops a mum in the course of the day.