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.)
But there's a catch. Even though this law is in Yoreh De'ah, it's neither widely publicized nor widely talked about. In fact, I'd never even heard of the law's existence until I was in my thirties, when I saw a certain comment by Mi Yodeya moderator Double AA.
In general, is the above law still applicable in America nowadays?
See also this related question.