Chol Hamo'ed, the "Intermediate Days" of Passover and Sukkot, have many laws of their own. Some holiday laws apply, others do not. Basically, labor is forbidden that is not required for Chol HaMo'ed, or the final days of the holiday. (The main exception is something that if not done, one would incur a loss).In the Talmud, we read that one may not write even a single letter in the Scribal Script. However, if one needs a mezzuzah or Tefillin for Chol Hamo'ed itself, one may write them. This at least implies that Tefillin are to be worn on Chol MaMo'ed. (Tefillin are not worn on Shabbat or holidays, but the status of Chol Hamo'ed is ambiguous). Based on this, the custom in nearly all communities was to wear Tefillin on Chol HaMo'ed. Many objected, however, as many statements in the Talmud represented a particular opinion, which may not be the final halachah. There is, in fact, an opinion in the Talmud that Tefillin are worn EVERY DAY, including Shabbat and Holidays. The above statement that one may write Tefillin for Chol HaMo'ed use, may reflect that minority view. The Vilna Gaon, for example, ruled that one must NOT put on Tefilin on Chol  Hamo 'ed. Many communities changed their custom with the publication of the Zohar (c. 1300). The Zohar considers the wearing of Tefillin on Chol HaMo'ed to be a great sin, as one is treating the holiday with contempt, making it like a weekday, for which loses one's share in the World to Come. Nearly all Sepharadic communities stopped wearing Tefillin on Chol HaMo'ed because of this, as did Kabbalistically inclined Ashkenazim. With the rise of Hassidism in the 18th century, the Jews of large areas of Eastern Europe ceased wearing Tefillin on Chol HaMo'ed. The question of what to do in the case of a conflict between Kabbalah and Halachah comes into play here, with the additional factor of the existence of opinions that even without the Kabbalah, there are ample opinions against the practice (Maimonides can be understood both ways. This is most unusual for his generally clear writings.) The rundown on who does what is as follows. German Jews put on Tefillin. Lithuanian Jews put them on, but, because of the doubt, do not recite the blessing. Those Lithuanian Jews who follow the Vilna Gaon, do not put on the Tefillin. Virtually all Sepharadim and Hassidim do not. Yemenites do not. In Israel, the accepted practice is not to put them on, although some do so quietly and discretely. It is not proper to have, in the same synagogue, some putting them on and some not. However, in "mixed" communities, this is usually not stressed. One rabbi I knew, in suburban New York, wanted to make two separate services for those who do and those who don't. He was fired for "wanting to divide the community". Although most people do whatever their father did, I consider that an error. It is more a question of ideology; to what extent we give weight to Kabbalah in deciding halachah. The commonly heard ideas that if one goes to work on Chol  Hamo 'ed one puts on Tefillin, and if not, he doesn't, is pure folklore with no halachic basis. I do not put on Tefillin on Chol HaMo'ed