According to the Mishnah on today’s daf (=page), there are four periods in the year when someone who sells an animal to another must inform him, “Today I sold its mother to be slaughtered,” or “Today I sold its young to be slaughtered.”
These days include:
- On the eve of the last day of Sukkot,
- On the eve of the first day of Passover,
- On the eve of Shavu’ot, and
- On the eve of Rosh HaShanah.
The reason for this law is that on these days many people slaughtered animals in preparation for the holiday, so it is understood that the purchaser has bought the animal for the holiday. If it cannot be on that day for that purpose, the purchaser would have to be told. Similarly, according to the Mishnah, if the seller sold a mother animal to the family of a groom and its offspring to the family of the bride he would have to inform them, since it is likely that they will be planning to slaughter the animals on the same day.
Tosafot quotes Rabbenu Tam as suggesting that on Sukkot people only prepared meat for their meals on the last day of the holiday because at the beginning of the holiday they were occupied with the other commandments of the day – lulavand sukkah. The last day – Shmeini Atzeret – is considered a unique holiday on its own that was viewed as a day that celebrates the special relationship between God and the Jewish People (see Masechet Sukkah, daf 55b).
The reason for concern prior to Yom Kippur, would appear to be based on the Gemara in Masechet Yoma (daf 81b) that teaches that someone who eats and drinks on erev Yom Kippur is credited as though he had fasted on both the ninth and the tenth days of Tishrei. This is generally understood to mean that there is a special mitzvah to eat on the day before Yom Kippur. Tosafot explain that although this would appear to apply everywhere, it was only in the Galilee that people ate meat; in other places people preferred lighter fare, like fish or poultry.