Yoreh De'ah 148:1 says, in part:
שְׁלשָׁה יָמִים לִפְנֵי חַגָּם שֶׁל גּוֹיִים עוֹבְדֵי אֱלִילִים אָסוּר לִקַּח מֵהֶם וְלִמְכֹּר לָהֶם דָּבָר הַמִּתְקַיֵּם. וּמֻתָּר לִמְכֹּר לָהֶם דָּבָר שֶׁאֵינוֹ מִתְקַיֵּם עַד יוֹם חַגָּם, כְּגוֹן יְרָקוֹת וְתַבְשִׁיל.
R' Pesach Feldman translates:
Three days before the festival of idolaters, one may not buy from them, or sell to them something that lasts. One may sell something that will not last until the day of their festival, such as vegetables or a cooked food.
To learn about the reasoning behind the law, see here.
Even though the law is in Yoreh De'ah, it's not widely publicized.
Does this law apply in America nowadays?
(Related question: "If a local store holds a 'Christmas Sale' in the days leading up to Dec. 25th, may I shop there?")