Answer

Hello.
I only just brought this as an example of a custom that uproots Jewish law (maybe that’s why you asked?!).
Our custom is not to wear them. Apparently this is because Hol HaMoed too is included in the “sign” that exists in the festival. I don’t have another position that would require me to deviate from the custom. Clearly there is no prohibition involved.
——————————————————————————————
Amir:
In the Beit Yosef on the Tur, Orach Chayim sec. 31, there is a detailed discussion of the matter, and among other things it says there that among the Sephardim they always followed the opinion of the Rosh and wore tefillin on Hol HaMoed, until the book of the Zohar became widely known, where it says that one should not wear tefillin on Hol HaMoed, and that this is even very severe. The Beit Yosef follows his usual approach that as long as the Zohar does not disagree with the Talmud, we rule in accordance with the Zohar, and therefore the Sephardim stopped wearing tefillin on Hol HaMoed

Although the Rema rules like the Rosh and the other medieval authorities who rule that one should wear tefillin on Hol HaMoed, the Hasidim, similar to the Sephardim, stopped wearing tefillin on Hol HaMoed following the words of the holy Zohar, and their opponents too, the students of the Vilna Gaon, stopped following the Vilna Gaon, who ruled not to wear them—but not because of the words of the holy Zohar, rather because he agreed with the medieval authorities who argued that Hol HaMoed too has a “sign,” and therefore one should not wear them

Since the customs of the students of the Vilna Gaon became the foundational custom of those who pray according to Ashkenaz rite in the Land of Israel, and those who pray according to Sephard rite follow the rite and prayer customs of the Hasidim, it comes out that here in Israel all Ashkenazim customarily do not wear tefillin

Outside Israel, those who pray according to Ashkenaz rite [those who do not follow the customs of the Vilna Gaon] do wear tefillin on Hol HaMoed in accordance with the Rema, and remove them before Hallel—I heard that here in Israel, in the central study hall of the Erlau Hasidim in Katamon, they do wear tefillin on Hol HaMoed, like the non-Hasidic Ashkenazim abroad—I haven’t seen it with my own eyes, but I heard that this is the custom there [this is a Hasidic group whose rebbes are direct descendants of the Chatam Sofer, who combine the path of the Chatam Sofer with the path of Hasidism, and in various matters they do not follow the same customs as other Hasidic groups—for example, in their central study hall they pray according to Ashkenaz rite and not according to Sephard rite, the rite of the Hasidim]
——————————————————————————————
Rabbi:
More power to you for the overview.
——————————————————————————————
Amir:
Not at all—the pleasure is entirely mine (-:

The issue of tefillin on Hol HaMoed specifically shows, in my opinion, not a custom that nullifies Jewish law but the opposite: a widespread practice throughout the Jewish world that, because of the spread of the holy Zohar, began to be abolished in entire communities—all the Sephardim and the Hasidim [as mentioned, the students of the Vilna Gaon, because of whom the “custom of the Land of Israel” became established, do not wear tefillin on Hol HaMoed not because of the words of the holy Zohar]

I don’t know if there is another such major change in actual practiced Jewish law due to the influence of the holy Zohar as in this issue
——————————————————————————————
Rabbi:
I didn’t understand.
Putting on tefillin on Hol HaMoed was a law (the regular commandment of tefillin) and not a custom. If so, what was nullified here is Jewish law, not custom.
However, one can discuss what nullified this law: was it a law / halakhic ruling that nullified a law, or a custom that nullified a law? Seemingly, for those who follow the Zohar, this is not Jewish law but custom. But those who follow the Vilna Gaon—this is a halakhic approach and not a custom (for he rules that the law of “sign” includes Hol HaMoed). True, this is a novel halakhic approach (as far as I recall it has no source in the Talmud). And if those who follow it do not themselves think this way but do so only because the custom was to follow the Vilna Gaon, then that too is custom and not law.
I already wrote here once that one should distinguish between two kinds of custom: there is a halakhic custom, meaning a custom to rule like one opinion or another (for example, the Shulchan Arukh or the Rema), and there is something that is entirely just custom (like kapparot or tashlikh). And there are important practical differences between them. For example, a halakhic custom does not obligate someone who holds otherwise (for that is his halakhic view), unlike a custom that is not halakhic.
——————————————————————————————
Amir:
Indeed, it may be that I wasn’t precise enough, and when I said “widespread custom” I meant more what you called a “halakhic custom,” because they followed the medieval authorities [the Rosh and those with him] who ruled that yes, one should wear tefillin, and not other medieval authorities such as the Rashba and those with him, who ruled not to wear them because Hol HaMoed too has a “sign,” and the Vilna Gaon followed them.

I just didn’t understand why you said that those who follow the Zohar on this matter—what they follow is called “custom” and not “Jewish law”; after all, the halakhic decisors do indeed see the Zohar as a halakhic source too, some more and some less, but there definitely is such an attitude toward it, no?

P.S.

1. A broader discussion in this specific case regarding the view of those who follow the Zohar, and an analysis of the Vilna Gaon’s position, can be found in Igrot Moshe, Orach Chayim vol. 5 sec. 24, paragraphs 6–7, for those interested in studying the subject
2. The issue of not wearing tefillin on Hol HaMoed among Ashkenazim in Israel is a classic example of what is called the “custom of the Land of Israel”—customs practiced by Ashkenazim in Israel that were changed from the original Ashkenazic custom of the Diaspora by the students of the Vilna Gaon who immigrated to the Land [many of these customs belong to the non-Hasidic Ashkenazim and those who pray according to Ashkenaz rite, but some of these customs whose source is in the students of the Vilna Gaon were also adopted by the Hasidim and the other worshippers according to Sephard rite, and I can’t go into it here at length]. Some of these customs are based on the opinion of the Vilna Gaon, and that is the case here, while in other cases they stem from Sephardic influence, since when the students of the Vilna Gaon came up at the end of the 18th century and the beginning of the 19th, most of the Jews here in the Land were Sephardim. This raises the discussion of why these customs are universally regarded as legitimate changes, while in other cases people bang on the table and declare, “The custom of Israel is Torah.”

In any case, for those interested in the topic, it is recommended to read an interesting discussion about the validity of the “custom of the Land of Israel” and to what extent it is binding or not binding even on Ashkenazim who wish to preserve their original Diaspora custom—the discussion is between Rabbi Yoel Katan and Rabbi Binyamin Shlomo Hamburger in volume 4 of the journal “Yerushateinu” of the “Ashkenaz Heritage” institute

The journal can be ordered on the Otzar HaChochma website
——————————————————————————————
Rabbi:
Every law needs a source. A Torah-level law needs a verse or its interpretation, and a rabbinic law needs an enactment of an authorized religious court. A practice in the Zohar that is not anchored in a verse or interpretation is not Jewish law—unless you view the Zohar as a religious court instituting something.
Even if something appears in Maimonides or the Shulchan Arukh without a source, it is not Jewish law (like the customs of Krakow that appear quite a bit in the Rema). This teaches you that the book itself is not what creates the law.
——————————————————————————————
Amir:
I understand …