For the source text click/tap here: Avodah Zarah 7
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An even stricter prohibition of business before idolatrous festivities belongs to Rabbi Ishmael - three days before and three days after. According to him, doing business with the worshippers of the Sun, who dedicated every Sunday to the God of Sun, is always prohibited.
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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.