Avodah Zarah 2 - No Business With Idolaters Before Their Celebrations

During the three days preceding the idolatrous festivals, it is forbidden to transact business with the votaries of this order, to lend them money or to borrow money from them, to repay a debt to them, or to accept payment of a debt from them.

Three days prior to his festival, the idolater is already focused on his deity, because at this time he begins to search for an animal that is suitable to use as a sacrifice. He therefore is likely to pay homage to his deity on the day of the festival for any benefits that have accrued to him from the point onward. A Jew therefore may not do business with an idolater during those three days, since he will indirectly cause the idolater to praise the name of his alien god.

Rabbi Yehudah says, "We accept payment from them, since this causes the payer distress," but the Sages reply to him, "Though it causes him distress now, he rejoices afterwards."

Art: Francisco De Goya y Lucientes - The Sacrifice of Pan