Avodah Zarah 6 - Why Three Days?
The prohibition to trade with idolaters begins three days before their holidays. But why is this? For example, one who sells an animal and its offspring before one of the Jewish holidays must inform the buyer, so that he does not transgress the prohibition of slaughtering a mother and its young on one day - and this prohibition is only one day before the holiday! - The difference is that this animal is purely for eating, but the idolater's case is for a sacrifice.
Then compare it to the law that one must learn the laws of a Jewish holiday thirty days before the holiday! - In the case of an idolater, the only disqualification is if the animal is missing a limb, and three days are enough, but in the Temple there are many animal disqualifications.
So the idolaters took the law of sacrifices to God and applied it to sacrifices to their idols, and are careful that the animal should be intact. What is the source for this law? - Noah was commanded to take whole animals into the Ark, because he would bring some as a sacrifice. But maybe they were not whole? - The Torah says, "With you," meaning, "Like you." But maybe Noah himself was not whole? - That can't be, because then only lame animals would be allowed.
Art: Jan van Balen - Noah And The Ark