פירוש על עבודה זרה 10:5

The assumption here is that it is prohibited to sell an animal to a gentile before his holiday because he might slaughter it that day and use it for a sacrifice. But why do we need three days before. A mishnah from Hullin (5:3) teaches that four (or five, according to one opinion) times a year a person can assume that an animal being sold today will be slaughtered today. Therefore, if he is selling a mother he must let the buyer know if the young had been sold to be slaughtered on that day. This is in order to prevent one from accidentally transgressing the prohibition of slaughtering a mother and her young on the same day. But again, the requirement is for one day. One can sell a mother on Tuesday if its young was sold the day before, because the two animals will not end up being slaughtered on the same day.