Chullin 83: Slaughter of Mother and Offspring on Erev Chag and Yom Tom

Today's Mishna teaches us that there are four times each year when a seller must tell a buyer that "Today I sold its mother to be slaughtered" or "Today I sold its offspring to be slaughtered".  These days are erev, evening at the start, Sukkot, erev Passover, erev Shavuot, and erev Rosh HaShana.  Rabbi Yosei HGelili adds erev Yom Kippur when in the Galilee.

This is about volume.  When so many people are buying animals for slaughter, it would be simple to sell a mother animal to the family of a groom and its offspring to the family of a bride.  They would have to be informed because otherwise a mother and its offspring would be slaughtered on the same day.  

The Gemara offers possible reasons behind each of these four days.  Regarding erev Yom Kippur, Masechet Yoma (81b) teaches that one who eats and drinks on erev Yom Kippur is seen as though s/he fasted on the ninth and tenth of Tishrei.  Thus it is a special mitzvah to eat and drink before erev Yom Kippur.  This applies especially in the Galilee where people ate meat, and not lighter meals like fish or poultry.