פירוש על עבודה זרה 14:2

Rabbi Ishmael states that not only is it forbidden to make transactions with non-Jews during the three days before a pagan holiday but it is also forbidden during the three days following the holiday. In the Palestinian Talmud two potential reasons are given for Rabbi Ishmael’s statement. The first is that non-Jews continue to celebrate for three days after their holidays are over and therefore these three days are also forbidden. The second is that if the non-Jew knows that he will not be able to conduct business with the Jew after his holiday, he will be depressed during his holiday and he will engage in less idol worship.
The sages prohibit only the three days preceding the holiday and not the three days following.