What is the single most widely practiced minhag (religious Jewish practice that's not mandated by halacha) among Jews in the United States today? By "widely practiced" I mean practiced by the most people.
I think there are two questions here:
- What custom is most widespread among observant American Jews that is less common in other countries?
- Name something post-Talmudic that most American Jews, observant or not, keep.
For the latter category, I think we could point to the Jewish customs of mourning, which are observed overwhelmingly across the spectrum of observance. Most notably, saying Kaddish! (Post-Talmudic.)
For a custom that is, let's say, post-Shulchan Aruch, I would say covering the mirrors in a shiva house. Some say this was simply a matter of covering the mirrors in rooms where prayers would be held (we don't pray facing a mirror), but Rabbi Joseph Dov Soloveichik argued that this was a version of a Talmudic custom regarding repositioning all the beds in the house -- they're both ways of expressing the loss of humanity experienced.
Are we including minhage sh'tus? How about gift-giving on Chanuka?
https://judaism.stackexchange.com/questions/3572 — apparently it might not be a minhag sh'tus.